The way in which the new captures are "written" in to the summoning selection text is not regular. For instance, my current list looks like:
1. Wizard (2nd most recent captured)
2. Hraithe (first thing ever captured)
3. Haakai (most recent captured)
4. Drake
5. Shambler
6. Gazer (4th most recent captured; replaced Cryodrake!) :-(
7. Fiery Demon
8. Lich (3rd most recent captured
In other words, my last 4 captures were written to slots 6, 8, 1, and then 3. Expected order would have been 8, 1, 2, 3. I'm not distraught because I still have Wizard, Lich, and Haakai, which honestly are probably the best in most situations, but thought I would report this behavior here anyway.
Edit: I just captured another Cryodrake and it replaced the Drake in slot (4) as expected.