What truly evil man is responsible for the prominence of the variable y in mathematics? The letter's "tail" gets in the way of the numbers and symbols on the line beneath it. And why dx and dy, rather than Δx and Δy?
Delta is for non-infinitesimal differences, rather than differentials.
(Well, also for other things. Actually, the only thing I can think of at the moment is the diagonal: Δ(X)={(x,x): x ∈ X} ⊆ X × X.)
Deltas are used for infinitesimals in non-standard calculus.
http://en.wikipedia.org/wiki/Non-standard_calculus
