Iknow that this probably does not apply in this case, but it also happens that ATC is only human just like Pilots, therefore when I am being given a partial clearance or instructions without explanation (i know this because I frequently fly the same route) then it is possible that i might push the envelope because i don't like to be the patsy all the time.
For example: If AtC of a certain area that is dominated by one large carrier issues instructions to us like: After passing xxxxx fly hdg 340 for entrail spacing and then the guy 4000ft above and 10 miles behind us with the two big AA's on his tail, is cleared direct to XXXXX descend PD to cross XXXXX 10000 @ 250kts then i know that the explanation for my instructions was something like: you are #2 behind the guy behind and above you, his company does not like him to be late and they have a hell of a lot more clout than you little guys do.