Reptile legs usually add length in a direction that is orthogonal to the booper-to-pooper one.

Cotton Hill, is that you?

kicks Hotwheels Sisyphus under the rug

Perhaps their tails are easily or often damaged?

So measuring from full fore to aft might give inconsistent results among other similar variables for the examined species, such as location, age, and weight. Going booper to pooper might give more reliable data, if the tails are often snipped at the tip by other reptiles, predators, disease, rogue mohels, swamp boat propellers, or hastily closed doors.

This is my best guess, I'm not lizardologist or a measuresmith.

I mean, it's not the exact same, but human fetuses can be measured similarly with a "crown to rump" length. I have to notate this in my reports (but I include other measurements too like hand/foot length and "crown to heel" length).