There is usually an oil in these systems mixed in with the refrigerant, so I don't really buy the "bubble" hypothesis.

A tiny amount of water vapor in the system has a HUGE effect on the performance of the refrigeration cycle. This is because water has an enormous latent heat of vaporization (which is the same reason you should turn on your AC and let it run all summer without opening the windows if you live in a humid climate). Anyhow, a contaminated system would do this, and then perform well once it was evacuated (all air/water vapor removed) and recharged. Of course, the system *became* contaminated with humidity because there was a slow leak in it, and eventually the performance would degrade.