maybe they have more vPro and/or phone-home stuff baked into their BIOS ?
Discussion
Possibly, but the CMOS battery is only supposed to be used when the system is powered off so that the BIOS retains its settings. It is not supposed to be used otherwise, if I understand correctly.
Perhaps that is a design flaw in Dell computers and it is being used some while one, or more than other brands, and thus causing CMOS batteries in Dell computers to fail more quickly.
It'd be fun to talk to a Dell engineer sometime.