TDEE - 20% question

mrp56839

I know this conversation is like beating a dead horse, but I'm not sure I understand why there's a mismatch or why TDEE-20% is used so much.

I used the frog TDEE calculator website that I've seen around here.

5'7"
190
34yo F
Desk job, just to take activity out of the conversation.

TDEE says 1957.
TDEE - 20% is 1565 per day
That's a 2740 cal deficit every week.
about a .75lb loss every week.

TDEE *7 days = 13699
less 3500 cals (1lb)
= 10199 cals per week to lose 1lb, or 1457 cals per day.

TDEE *7 days = 13699
less 5250 cals (1.5lb)
= 8449 cals per week to lose 1.5lbs, or 1207 cals per day

My MFP is set for 1.5lbs/week and the 1207 calculation is pretty close the the 1250 they've calculated for me. (I've actually been thinking of raising it to 1500, but to keep with 1lb, it looks like that's a little high.)

So my question is why do people recommend TDEE-20% if that doesn't create at least a 1lb deficit?