This is the helpful flag count on my profile:
And this is the one on my nomination
Which is quite different. Is this normal and am I overlooking something obvious or is this some sort of bug?
It's normal. It's caused by differences between flag types.
As you will see if you click the helpful flags number on your profile, there are 4 categories of flags (shown on the right sidebar):
This category is basically defined as a flag which doesn't fit into any other category.
Any comment flag is grouped here, even spam/offensive comment flags, as they're all handled by ♦ moderators.
Your helpful flags count on your profile will display the total helpful flags of all the above varieties.
However, the helpful flags on your election nomination only displays the helpful flag count for "normal" flags.