Sandhi is a real thing. (Source: I had to study this shit to teach pronunciation classes.)
It took me WEEKS to recognize that what I thought I was saying and what noises I was actually making are completely and utterly different. There's often no relationship (like "coodja" for "could you" or "chrain" for "train") between the intended sound and the actual sound ... but since everybody does it you don't notice until its forced into your face. The only time you make distinct sounds as per the "official" description (and even then not as often as you think: I submit "train" once again as evidence) is when you're deliberately speaking slowly and distinctly. Which is almost never (and comes across as condescending in actual interaction).
Weeks, I say again. WEEKS. And this was under constant training that included the playback of what we'd actually said showing us what we were doing. The denial is embedded deeply in our psyche.