The charging action is different between a wet cell or gel type battery and the absorbed glass mat (AGM) battery. The wet/gel cell accepts the charge at a standard rate up to full charge, but the AGM rate of charge changes as it gets closer to full charge. I did hear from Nathan at XS Battery via e-mail since the forum was down this morning. He asked a couple of questions, and advised the following:
There are a few questions I have. Did it just start doing this and have you been charging it the same way all the time or did you change something? [Started about two weeks ago, nothing changed with the hookup. It did go to steady green (float mode) once, but after the next start has never done it again.]Is the battery hooked up in the car when you are trying to charge it? [Yes, the battery is in the car and hooked up.]

Sometimes with the Battery Tenders the cold weather can have an effect when trying to charge the battery. The Tender is such a low amperage that if the battery drops down in voltage much it may not be able to charge the battery fully. It can maintain the voltage for storage above 12.6V, but may not be able to top it off with a full charge.
The answer may be to start and run the car to boost the charge rate via the alternator for 1/2 hour or so. Weather does not support getting out for a drive right now