Between the ankles and the neck, there are well over a dozen joints and connections in the Obitsu skeleton. Many of them are where a long "bone" section fits into a joint. These can come out from their sockets a bit from the frequent rotation they receive. Another place to look at is the torso. Misalignment of the various interior torso parts (the "spine") can lead to slight height differences. Another place to look is the round abdominal part, a.k.a. the "dumb bell" (
http://parabox.jp/obitsu/60PARTS/60RP-F01NP-24.html). This little assembly forms the center piece of the 47/55/60cm Obitsu's spine. It consists of two parts that sleeve together and are held in place by a set screw. By loosening this screw the two parts can be slid in and out giving a small amount (1cm to about 1.5cm I think) of height adjustment.
Judging from your photo, it would appear the girl on the right has a little less of the top of her abdomen vinyl sticking out below the bust vinyl. Quite possibly the girl on the left has her round abdominal part extended a little, increasing her height.
Since both of your girls were bought "previously enjoyed", it wouldn't hurt to do a little simple disassembly to check for wear, clean house, and realign those spinal parts (the two oblong abdominal parts and the round abdominal part).