I was thinking:
At least in the first example, the words that use the ϭ symbol for E, are words that use silent E's. The word Cress, which uses Δ doesn't.
(I know those aren't the exact symbols, I just used the closest matches I could find.)
My guess is that different symbols might mean different pronunciations. The only problem with that is the other worlds like "Phantomile" which uses a Δ, even though it's silent.