Vendor Plans Calculating Incorrectly

Sure I posted this issue a week or so ago but it seems to have disappeared!

We have MVE 4.9.3.SP1, and since around 4.9.2 it's been incorrectly calculating vendor commissions.

For example:

We have an order in the system which is as follows: Goods £33.00, Delivery: £8.95, Tax: £8.39, Total £50.34

We have a vendor on a straight 8% commission plan, with shipping excluded from calculations.

CS Cart is calculating commission on this order at £2.76 + Tax, but it should be £2.64 + Tax. (There's a bit of rounding going on but it's far too big a difference to be that).

The weirdest thing though is that for different vendors on different plans, two orders may have the commission calculated at different rates (even though the plan doesn't change). Just can't get my head around it!

Any ideas?