So C0 really is the first bank. Not that it is important, but I would like to know why they start with C.
On the cartridge, the A23 pin (the one that controls the $80 bit of the bank) basically goes nowhere, meaning that banks $00-7F effectively mirror $80-FF as far as the cartridge is concerned. In fact I'm not even sure A23 is connected to the cartridge at all. It might not be... I'd have to check a pinout. Regardless, it doesn't really matter.
(EDIT again: yes it looks like all A pins connect to the cartridge. Though the doc I found calls the bank lines BA0-BA7 instead of A16-A23: https://www.caitsith2.com/snes/flashcart/cart-chip-pinouts.html
However, on the SNES side, A23 allows for FastROM to be enabled. So memory accesses to banks $80+ can
be performed faster than memory accesses to banks $7F- (assuming the ROM is fast enough, and assuming FastROM is enabled). So basically, there's little reason to not default to banks $80+ since FastROM can be software controlled.
As for A22 (the $40 bit), that needs to be set for HiROM because memory accesses only go to the cartridge if either A22 or A15 are high. Meaning $80/0000 doesn't go to the cartridge because both A22 and A15 are low... however $C0/0000 does because A22 is high.
banks $00-3F = slow, only upper half of bank goes to cartridge
banks $40-7F = slow, full bank goes to cartridge
banks $80-BF = fast, only upper half of bank goes to cartridge
banks $C0-FF = fast, full bank goes to cartridge.
So yeah -- HiROM games will generally use banks $C0+ because there is little reason not to.
EDIT 2: Also WRAM hijacks banks $7E/7F, so you have 2 more banks in the $C0-FF range than you would in the $40-7F range.
I think I got it now. Just for verification, E3/4000 would correspondent to 2x34000, right?
$E3 & $3F = $23
So 0x234000, yes.
(the '0x' prefix on numbers just means the number is in hexadecimal. It's the same as the '$' prefix. I typically use '0x' specifically for file offsets and '$' for addresses to differentiate them, but they mean the exact the same thing)
EDIT: whoops, A15, not A21. Sorry just woke up.