This is a curiosity question. The batteries tested were Duracell coppertop alkaline batteries.

Recently, my bass guitar was acting weird and it was caused by a weak 9V battery. I checked the voltage and it was like 6.5V or something crazy low. Now, if I check a C 1.5V battery when it's weak, its voltage is relatively high. I think I measured the weak C at 1.42V.

I forgot to check the 9V while under load and I wasn't able to measure the voltages of the C battery under load b/c all the devices I could find needed the cover closed for it to work. 

In any case, why does a 9V drop that much when it's weak but a 1.5V doesn't?