Was Professor Snape's handwriting significantly different from the Half-Blood Prince's?

Question

It occurs me that it's a bit strange that after 5 years of being Snape's students, and presumably seeing his handwriting almost everyday - writing class instructions, correcting papers etc, neither Harry, Ron, nor Hermione could compare the HBP's handwriting to Snape's.

The book mentions HBP's writing as being "small ...[and] cramped". Is there any description of adult Snape's handwriting in the books, or any discussion in interviews and the like regarding why none of the trio (especially Hermione) recognized the handwriting as being Snape's?

Answer 1

Here's another example describing Snape's handwriting at age 15:

Harry moved around behind Snape and read the heading of the examination paper: DEFENCE AGAINST THE DARK ARTS – ORDINARY WIZARDING LEVEL.

So Snape had to be fifteen or sixteen, around Harry’s own age. His hand was flying across the parchment; he had written at least a foot more than his closest neighbours, and yet his writing was minuscule and cramped.

_{Order of the Phoenix - pages 564-565 - Bloomsbury - chapter twenty-eight, Snape's Worst Memory}

A few things:

When it comes to a canonical characterization (I believe as you seem to), Hermione ostensibly should have noticed, if Snape's handwriting remained the same from his teen years to adulthood -- this is extremely unlikely. There is nothing in canon that directly addresses this.

J.K. Rowling needed Hermione to not notice because JKR needed us not to know who the Half-Blood Prince is until the story is wrapping up. Harry needed to be the one who discovers the truth to the secret of the Half-Blood Prince.

As DVK and alexwlchan both mention, handwriting can change significantly between the teen years and adulthood. Some similarities exist (see out-of-universe example below) but the finished products could easily be mistaken for separate writers if one isn't looking with a super keen eye.

TEEN:

ADULT:

Same writer.

Answer 2

I can’t find anything describing the adult Snape’s handwriting (books, Pottermore, interviews, nada). But here’s one passage which seems relevant:

Although Harry had offered to share his book with both of them, Ron had more difficulty deciphering the handwriting than Harry did, and could not keep asking Harry to read aloud or it might look suspicious.

I would expect that after five years of his criticism on their homework, they’d be sufficiently familiar with his handwriting that they wouldn’t have much difficulty reading it. This suggests that Snape’s handwriting as a teenager and as an adult are quite different.

I know that’s pretty weak sauce, but it’s the only canon source I can think of which comments on the legibility of the handwriting in the textbook.