Video compression. Another separate modern marvel of mathematics.
Not sure what security cameras you’re using, but mine all use H.264 1080p compressed streams. I get about 15 days worth on a 2TB spinning rust across 6 cameras.
Even going by your own numbers, you almost certainly are using compression too. 9.32 days on 2TB does not get you a month on 10TB if you “have a mix of 4K & HD”.