The fact that the harmonised vowel is always front and unrounded is presumably related to a pronounced-but unwritten-epenthetic vowel that occurs between ‹б› and ‹ль› in the bare stem forms.
since no vowel is inserted in forms with a following vowel (e.g., ансамбли, рубли), this phenomenon provides an interesting case of phonological opacity-an analysis of which is beyond the scope of the present paper.