Weekly size for baby, different between apps?

By 20 weeks the baby is measured head to toe now. Maybe on What to Expect they still have crown to rump measurement 

Glow goes by head to feet for length, what to expect still goes by head to rump. 

Anything is just a guesstimate. But I did read that around this time they are able to start measuring from head to toe, instead of crown to rump- so perhaps that's where the large difference comes from. This is just my theory I have no clue if it's correct. I kinda take the measurements with a grain of salt, as I have 4 different apps that all say something slightly different. 

Well considering everyone grows at different rates, the measurements they give you are just estimates anyway. The only way to get a good sense of how big your baby is is from ultrasounds. For example, at 17 weeks my baby was measuring about an ounce more than what glow estimated, and I saw another Golw user say her baby was about 4 ounces larger than what glow estimated. I guess if you're getting a discrepancy in sizes from the apps, I would just average it out and take that because it's not that accurate anyway.

Everything will be different than actual size glow says baby girl should be 6.7 ounces, but my last ultrasound she was measuring 7 ounces. These apps I believe just give an estimate based on statistic wise.