I do this too - "thee actual" vs "thuh real" and such. I do expect it's to do with having a vowel sound of a certain kind at the beginning of the next work making you move your tongue in a certain way.

with "the actual" I imagine you're also putting in a kind of intrusive "y" sound so it flows better into the vowel

yep although I don't think it sounds too intrusive. it comes out as theeyactual while the alternative would the thuh.Actual with a sort of micro-accent on the A. I'm sure linguists have names for all these things, like when the movements of the tongue and mouth incidentally form phonemes!