You forget 20GB = 1024*20=20480MB
So the price for per MB for the 20GB bundle is 14.99c.....i.e. cheaper than 15c/MB
Same story for the other larger bundles.
That said, it makes sense to use a small bundle like the 500MB one and just use OOB at 15c/MB afterwards.
Ah yes, you are right I did forget that CellC uses 1024. Yeah I'm probably just going to get a R15 100mb bundle instead of the R89/100mb I was paying on MTN. If I use more then so what. Still cheaper to use my 8ta 10gb on wifi but not a train smash to use a gig on my phone. (already saving about R150 just on the calls moving to cellc )