For me it seems that N900 reports battery charge level relative to original design capacity and not current capacity. Battery capacity is decreasing over time (battery used or not) and I think that even in new devices it's not 100%. Maybe that's the reason why battery level indicator shows 100% even if lshal show only 80%. Indicator is calibarated to prevent user complaints (my battery doesn't charge to 100%!).

Nevertheless, I agree that 87% is low. In my 1 year old laptop, Ubuntu reports capacity at 90% (AFAIK it was about 95% when it was new).