1) It's possible Google's system somehow had miscounted previously.
2) It's possible they've now miscounted due to system error.
3) They may have changed what they're counting for this percent, and the count was updated when you updated the local business listing. For instance, they added Events not too long ago, so this new feature may now be counted as part of the percent.
I suspect #3 may be the most-likely explanation. You could try adding in an Event and see if the percentage changes.
I haven't seen this happen yet, but then most of my clients do not need frequent account updates. Also, I rarely care what a website reports as to a profile's completion rate -- for instance on LinkedIn, it doesn't affect you at all if your profile is only "75%" complete or somesuch.
Being at 100% completion in Google may not necessarily help you rank above competitors' listings. So, I'd recommend focusing on the quality of listing update content more than their reported percentage.