The short answer is that I don't know the answer, but would guess that it may have something to do with a known accounting problem in calculating percentages.
The best example of this is in calculating eBay's selling fees. When computers calculate payments due as a percentage the result will often be to a decimal point and not an exact penny.
If the fee due is say £6.09.42 the amount charged can only be £6.09. But the outstanding 0.42 of a penny remains on the seller's account. When enough decimal points add up to a full penny, it is added to the seller's next invoice. (EBay didn't get where it is today by overlooking fractions of a penny!)
The same thing could possibly happen to your PayPal account if you have incurred fees that are calculated as a percentage, for example making an exchange rate conversion or charging a currency conversion fee.
Only a guess, but I think it's the most likely answer. PayPal can't and won't gratuitously add a penny to the cost of your purchases!
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