Race results on companion app vs during race

I’m new to Zwift. In my race today when I crossed the finish line it told me I finished 54th.
However, when I checked my activity on the companion app, it states I finished 15th.

I was racing in group D. Is that 54th my placing across all the groups and the 15th my placing within group D?

You read it correctly, 54th overall (if you can see all cat racers in a race then the position will be overall in-game) and 15th in D. Register in Zwiftpower and you will find a bunch of people in your grade will have been disqualified anyway s they are racing in the wrong cat. Your w/kg avg in that race would suggest you are C grade by the way but if you register on Zwiftpower, it will keep track of your results and advise the right grade based on your results.

Hi Dean, thanks for the assistance.
I registered on Zwiftpower and it says I came 2nd out of 4 in group D. It only shows 29 people in total, so I guess it only displays those that have registered on Zwiftpower.

How do you know my w/kg?

Yep, not everyone registers but it seems that as you improve your Cat, so does the quality of fellow racers. For example, B grade gets far less cheaters and / or sandbaggers than D & C.

Generally speaking w/kg is power / weight which in your case is in that race is 189w / 72kg = 2.6w/kg but ZP bases your category on 95% of your 20min avg (which is 2.49w/kg for your only race recorded in ZP) . I’d suggest that you continue with D, and keep an eye on ZP results. When you start pushing that power or more consistently, you will start getting UPG (disqualified) and you will know its time to go to Cat C.