Westco battery is not really a vehicle battery. It is a gelled electrolyte battery with slightly different chemistry. As such, it has a higher resting voltage than a conventional wet-cell battery. The charging system on the bike probably operates at higher voltage than that from the battery tender so will be able to fully charge it. After running the engine for a bit, I expect the battery voltage is higher than the maintenance voltage of the tender. This is what produces the unexpected behavior you are seeing.
Leave the bike for a few days or turn the lights on for a while and I'll bet the expected behavior returns. You should really try to get a maintainer for the Westco battery but I don't think it will hurt to use what you've got.