I preheat mine and never, ever beat on them.
Jon, I think it's likely. It took me 25 years to realize it, but I get lazy with my grip hand while cutting the sprue and that can let the blocks shift a thousandth or so, which is usually about how much my bullets are out of round with anyone's mould. I mount all mine the same way typically, with the sprue plate pivot on the left block close to the handle hinge, and I tend to hook my middle and ring fingertips around the front corner of the left block to brace my hand while I push the sprue plate open with my thumb. Putting force on it that way pulls the sprues...and bullets....forward slightly in the cavities of the left block and if I don't have the handles squeezed tight, or turn the mould crosswise to my body and make a deliberate push straight across the left block with the sprue plate, this out of round thing happens.
Reversing the blocks enables me to put angular force on the sprue plate, actually pushing toward the pivot screw as much as straight across the blocks, to counteract the swing arc and direct the cutting stress at 90° to the block where the sprue plate is hinged.
I'm starting to understand why some mould makers mount their sprue plate pivots in the middle.