Literacy was not the most normal of conditions until fairly recently. There are parts of the world even today where it is uncommon. It is therefore no surprise that in former times, things were communicated more often by speech than by writing. However, the sense of spell to mean "writing out a word" developed via the verbal sense "to mean, to signify."
It is not strange that people would assume the meaning of magic spell to relate to the word spelling as this is the normal sense of the verb spell nowadays. These kinds of common sense etymologies are what are known as "folk etymology" -- where an obsolete, foreign, or otherwise unfamiliar word is replaced in popular understanding with a more commonly known word or sense. It can sometimes cause the spelling or meaning of a word to change.
The technical term "folk etymology" refers to a change in the form of a word caused by erroneous popular suppositions about the history of the word. Until the scholarly development of comparative linguistics and descriptions of laws underlying sound changes, which has mostly only happened in the last couple hundred years, the derivation of a word was generally determined by guess-work, often involving long and entertaining, but utterly incorrect, stories to explain the WHY. Speculation about the original form of words in turn feeds back into the development of the word and thus becomes a part of a new etymology.
Believing a word to have a certain origin, people begin to pronounce, spell, or otherwise use the word in a manner appropriate to that perceived origin. This popular etymologizing has had a powerful influence on the forms which words take. Examples in English include crayfish or crawfish, which are not historically related to fish but the name for which come from Middle English crevis, cognate with French écrevisse.
But there is another common mistake that people make, which is to assume one word is from another in the wrong order. A word used in magic that falls victim to this type of supposition is curse. The term "curse word" is not because such terms used to be (or still are) employed in the casting of magical curses. Rather the word curse meaning "a malediction or wish of evil" came first; later came by extension the sense of using blasphemous or profane language. The word cursive is unrelated.
Further mistakes, which can even lead one into danger, come from these assumptions that words have always been the same and have always held the same meanings. People fall into the trap of looking for "hidden meanings" in words or using words as a type of fortune telling. An alarming number of people don't even realize that it is fortune telling, and instead think that real etymology is done in such a manner -- that if a person is named Amelie then she must be a liar because lie is right in her name, or that a person waving a friendly hello is really wishing you to the depths of Hell (Hell+low). These kinds of beliefs are worse than superstition, as there is not even a valid "in my culture it is believed that X really means Y" for most of these, leaving them to be made up on the spot by the individual who is deliberately seeking out such "secret meanings" and who always chooses them to suit their tastes. It is remarkable that the alleged "secret meanings" discovered are always something negative -- I have never heard anyone claim to hear someone giving "secret blessings" but instead, every person who does this style of fortune telling thinks they constantly hear secret curses or warnings of impending danger (usually coming from a person they already dislike, and thus they are very pleased that all their suspicions are confirmed in having so astutely discovered this "proof" of the evil intentions.) I strongly discourage anyone from performing this sort of willy-nilly "discovery" of supposed word meanings, as it never leads to a correct answer and only risks stirring needless hatred and paranoia.
It is sadly common that in magic, and maybe (let's admit) other fields too, folks prefer to hear only that which confirms what they already know, rather than risk learning something new. But the best magicians are those who are happy to study and learn.