What is the difference in CPU utilization between 'ps aux' and 'ps -ef'?

The %cpu and C columns are showing almost, but not quite, the same thing. If you look at the source for ps in ps/output.c you can see the differences between pr_c and pr_cpu

C is the integer value for %cpu as you can guess. The odd difference is that C is clamped to a maximum of 99 while %cpu is not (there's a check for it for %cpu but it just changes the format from xx.x% to xxx%).

Now, I'm not really sure why C has this clamping; it seems a little arbitrary. It's been there since procps 3.2.7 (2006) so it probably was from the era of single CPUs