Pretty neat upgrade. I don't really understand why this is possible without additional fees, but glad to hear it is working anyway.
Basically, when you first enroll coins in to Whirlpool, it does so with what it thinks will be a safe fee for relatively quick confirmation of a 5-input-5-output coinjoin. If, when those coins actually come to be coinjoined, it turns out that fee is higher than necessary, it will enroll additional inputs in order to lower the overall fee rate back to a more appropriate level.
btw, is there a statistic showing how many coin-join tx whirlpool is running per day? Just curious to see how popular their service has become.
See the "Samourai Whirlpool" box at the top of the 4th column here:
https://bitcoin.clarkmoody.com/dashboard/. 7,657 transactions coinjoins over the last 30 days.