Using Refractometer to measure Carbonation Level

I use a Refractometer when I brew mostly for the boil off phase of an all grain batch. This helps me make sure I get to the exact FG I want for the specific type of beer. It works well and I don’t have problems.

One day while drinking one of my Märzen beers I wondered what the gravity of my beer would be now. Even though I’d taken the FG before racking and knew it was on point. So I took a few drops adjusted for temperature and took a reading.

My FG for this beer was 1.013 - I racked to keg and then force carbonated. What was the reading of my beer now (?) 1.025

I was shocked. This must be a mistake I thought - I knew the bubbles would increase the buoyancy and so the gravity but that would have a bigger effect on a hydrometer surely?

I’ve subsequently brewed and tested all through the forced carbonation process and yes the gravity rose. I estimate that a carbonation level of 2.5 - 3 gives you a gravity reading of 1.025 - 1.035.

Has anyone else experienced this? It is an adequate measure of carbonation levels? Where could one find out more about the relationship between gravity and carbonation levels?