Sitting in a spa (hot tub)

This may sound odd – it does to me – but it happened and I’m pleased : ) We bought a swim spa which I love and actually, I’ve been working out in it everyday (water aerobics and swimming – well my version of swimming lol, but I digress) but I could never sit in the seats. Everytime I sat, my body would float up out of the seat. I can’t describe it properly but it just didn’t work! Last night, I finished exercise and sat in the seat, and for the first time I didn’t float out/bob out of it. I just sat there like a normal person. So, then I sat in the other seat, and then the lounger. I was very excited about this and because I had not disclosed to my hubby my physical inability to do this before, he didn’t quite get my level of excitement lol!! Anyway, does anyone know how this works? I’ve read it could be that due to exercise I’ve increased muscle and possibly lost some fat. Whatever it is – it’s a NSV for me!