What does E mean in this instance?
"E" stands for "Electromotive force", which is essentially just voltage. We just have come used to using "V" instead of "E"

It would be the same as asking why Current is "I" even though it is measured as Amps.

Likewise it would also be the same as asking why Resistance is "R" even though it is measured in ohms.

This might also help you understand the saying "Eli the Ice Man" where the E stands for voltage.