What causes the variance in protein abundance levels?

Just saw a great talk by Mark Biggin at CSHL about some work he has done in quantifying the degree to which protein abundance differences depend on translation rate vs. mRNA abundance. It relates to data from Schwanhausser et al., in which the authors claim that most of the variance comes from differences in protein translation. They arrive at this claim by showing that there is a lot of variance about the fit line between protein abundance and mRNA abundance. Mark made the excellent point that any sources of error would also lead to such variance, thus leading to an overestimation of the role of translation rate in protein abundance. Once you take this into account, the role of translation is much diminished, with transcription making up most of the difference. Very cool! And important, I think...