I think it's just familiarity. I don't know a specific word for this phenomenon in relation to enunciation. The reason is simply that initially, speakers (or whoever they're talking to) may not be familiar with the term, so they say it more clearly.
Much the same thing happens in writing. When a writer introduces a new term, the first usage may be "quotated" or italicised to call attention to it. Thereafter he'll just write it unquotated, assuming you know what he means. Or if it's a long expression it may be written in full the first time, after which he'll use an abbreviation or acronym.
The only "technical" term I can think of to describe this is redundancy (sense 5a) which can be reduced when particular vocalisations / expressions are "expected", so they don't need to be so clearly differentiated from alternatives that aren't expected in the current context.