That isn't related to the IF function, if I understand what you are saying. It is Numbers trying to be helpful by guessing what it is you are trying to type. It can be highly frustrating at times, especially when editing a previously written or partially written formula and it adds commas or +'s that you did not type or thinks you are typing a function name and puts it in the formula.
For complex formulas that it keeps messing up I use "show formula as text", which turns all that "helpfulness" off. When in the formula editor, click the down arrow to the right of the "fx" to choose it. The setting is only good for the one time, there is no option to make it stay that way.