I think that it was due to the fee per transaction from paypal, because I asked the same thing of Zulgh a long time ago. I am not sure how paypal's fee structure works these days though so maybe it has changed since.

If you do 10 transactions of $5, you hit 10 fees. One transaction of $50 had a lower total fee, so CF got more net benefit from the same transactions. Not sure if it was a lot, but it was there.