Were those the only two abnormalities they mentioned on the bloodwork, or did they specify that everything else was normal? It looks like the sodium level isn't terribly
out of range compared to the potassium level. High potassium could indicate a few different things, but from what I've read it's normally coupled with abnormal kidney enzyme levels/renal issues or cardiac arrhythmia, which it sounds like they didn't mention before the neuter. Plus, there are usually other physiological or behavioral issues that you likely would have noticed before now to indicate something serious (diarrhea, fatigue, appetite changes).
Simple over-exercise can temporarily elevate K levels, too. Common culprits for high potassium are Addison's Disease, urinary obstruction, renal failure, and Diabetes.
It could also be a lab fluke or margin of error issue, too.
To ease your mind I would get a copy of the bloodwork that was done before the neuter to give to your normal vet and have him/her do a repeat just to be sure, but you're probably already on that.
There was a lot of interesting info out there when I was reading up on this, but this page
summed up the most pertinent points I kept running across if you're interested.
Hope all goes well.