FWIW, rechargeable AA cells have always had a lower voltage than alkaline disposables. I think this has been true across various chemistries over the years. No idea why, but it’s a fact of life. Most electronics that use AA cells are equipped to handle both alkaline and rechargeable cells. This includes RC electronics, which are typically designed to run on 5V, rather than the 6V you would see with 4x AA alkaline cells.
On the upside, rechargeable AA cells, such as NiMH, are less sensitive to low charge than our fickle LiPo packs. So setting the correct voltage alarm threshold may not be quite as critical.