Well sometimes they aren't typed and the letters could be cut out of magazines or something. If it was that way, it could be to keep their handwriting confidential. Now, if it was typed on a computer, I have no idea why they would change the font. Maybe just to make it pretty?
parts of it were taken from different magazines. The multiple magazines make it harder to tell who made the note.smc91352
Well its impossible to tell from the letters themselves, unless you can notice a pattern and narrow down who would have magazines with such fonts and where the person might've cut them out from. However, then you would have to figure out which issue of what magazine it would be from, although there could be similarities in fonts used by the same magazines for multiple issues. I suppose one could go about the fingerprint route, but you arent the police.
There are special units in police forces that can track people using their handwriting. It's pretty clear why people would want to disguise ANY trace of their identity. If they have to go as far as their handwriting, they have to do it.