I had a look at the source and there is a if/else condition in AP_MotorsUGV.cpp, which checks for turning on the spot without moving forward/backwards and turning while moving. The problem seems to be the 0.5 scaling factor for pivot turning. This causes a doubling of the output to one motor when the condition changes. This is not noticable with small inputs, but causes a jump in throttle output, when steering input is at its max.
I am no programmer at all, but I think it would be good to have a normal "elevon" mix for skid steering and make the scaling a configurable parameter. This way a wider range of vehicles would work. There are vehicles like the Dagu Wildthumper, which require a lot of throttle to do a pivot turn, or on the other hand, robots with two powered wheels and a free pivoting third wheel, which turn easily.