Now for lifting, FitBit is indeed going to be underestimating.
I'd recommend logging strength training in MFP and take that lowish calorie burn, which is actually better estimate, and it'll update FitBit correctly when synced. Should find the TDEE come up a bit depending on how much lifting is done.

I actually made an improvement in the spreadsheet, I didn't assume straight BMR, but rather average of BMR and RMR for 2/3 of the time, because actually during awake non-moving time you burn RMR, which is more than BMR. When sleeping you burn BMR (unless repairing from lifting then it's higher). So Katch BMR and Mifflin using that adjusted height should be off a tad, higher for adjusted BMR.

So if you had a BMR based on age, weight, height as FitBit uses, as weight goes down BMR would go down. Say 1500.
Those BMR formula's have assumption you have same healthy ratio of fat mass to lean body mass, and as you lose weight the ratio stays the same.
Rarely true until you come in to goal weight area.

So that's why spreadsheet asks for bodyfat %, and bases BMR on the LBM in your body. There is still an assumed ratio fat:non-fat, but since fat mass contributes very little to metabolism, having higher fat isn't off nearly as much as the other BMR calc can be. Say 1600 because you did lifting during the diet and held on to muscle mass.

So using Katch BMR, and eating at reasonable deficit, you can lose weight, fat weight, but your LBM stays exactly the same.
So your BMR actually stayed the same, 1600.

But FitBit and MFP using Mifflin BMR, see the weight goes down, that formula says your LBM went down, therefore the BMR went down, so they would adjust from say 1500 to 1400.

So now to tell FitBit your BMR really didn't go down, your height has to go up to increase BMR even more. So what took 70 inches to make the FitBit BMR match the Katch BMR, now takes 75 inches.

Because next to weight, height is the next biggest influence on BMR. It's actually surface area of the skin, but since that is very hard to come by, studies figured out what formula would be with height, tad easier to come up with.

Are you week 4 of cut or reset? I haven't heard of anyone losing weight on the reset so I am guessing your doing cut?

That's a big difference. What do the 2 measurement formulas say each, they can be 5% accurate.

First, calipers can be 5% accurate in experienced hands. Was that the case for recent estimate, because you can't do that shoulder spot yourself.

My mom did the measurements and then I measured her. We've been doing it off and on for about two years but neither of us is experts.

If you have the other measurements from the same time when you did the Bodpod, you can get an average formula estimate, and see what the correction factor for it would be to reach the better Bodpod reading.
So like perhaps at that time the average on the 2 formulas says 25%. So correction factor of 1.2.
Now formula's average say 22%, reality would be closer to 22x1.2= 26.4.

So really depends if you have some other estimate within a week of doing the Bodpod, that you can use again now.