It's not surprising at all. The 115 configuration has more speaker area and volume than a 210 configuration does, and the 15" driver is quite a bit more efficient than the 10" driver, even with two of them effectively doubling their efficiency.
All things being equal, the amp head feeding the 115 cab will be louder and more powerful than the same amp going into the 210 cab.
That's the reason why the Rumble 200 combo (1x15") and the Rumble 500 combo (2x10") are about the same volume with the internal speaker. The amp wattage is balanced out by the speaker difference.
Struggling to decide which extension cab to get. I have the 200c. First upgrade is the extension. Then trading the combo for the 500 head.
After reading yoir experience I'm goong with the 15. Then later a second extension. Probably the 210 for variety.
