Vector Checksum - STM32

I have a working program to blink an LED on an Olimex STM32-P103 board.

With gdb I looked at the vector checksum at 0x1c and it is all zeroes. But it runs OK.

So I patched the binary file to plug in the correct checksum and loaded that into flash. It also programs and runs, and gdb shows the checksum as programmed.

Can anyone explain why it makes no difference with or without the vector checksum?

Thanks,

Dave