Hrmmm - now that you ask, I have no idea. If it's to be found in canon, one might have to go back several editions - I remember it from about the time of 3rd-4th, at least. Or, just as likely, it might just be a house-interpretation that has become canonized in my own memory.
Surprisingly few canon examples of any spells with requisites exist - the canon spell examples are not so overly complex and crafty as those that many Players invent, tho' that is not to criticize the work of the authors. If we can see further, it's only because we stand on the shoulders of... erm... those who are giants compared to us.
Looking at the 5th ed rules (p 77-8), the first sentences describing Intellego are that it "is the Art of perception. It allows a magus to gather information directly from the forms of things. This information does not deal with the appearances of things, unless Intellego Imaginem magic is used..." While this, as apparently intended, refers to nature vs appearance in the context of "mundane disguises", it is not so far a leap from what we are talking about here, mundane changes in appearance.
The rationale, then, would be that a CrFo spell with In requisite and duration allows the magus to automatically monitor and then unconsciously adjust the creation - meh, ymmv.