HCG levels rising but not doubling? *confused*

I have 1 daughter who’s almost 3 and since we were blessed by her we’ve been trying for another! We have had 3 miscarriages after her in a row. The last was almost a year ago, we lost our baby boy at 15weeks. We found out last week *super early* that we are pregnant again!

I went in at 10dpo (wednesday 1/6, 4weeks) to get my blood drawn.

HCG- 16.4

Progesterone 13.7

Followed up by blood work at 12dpo (friday 1/8, 4w2d)

HCG - 49.5

I got these results today because of the weekend.

My doctor’s nurse called and said “your HCG is rising but not doubling.” So i’m going in tomorrow (Tuesday 1/12,4w6d) to get another HCG draw.

I’m just confused because isn’t that a doubled number?? What does this mean?