I believe the 460V rating is because the earlier delta services were usually 230V so we could have a dual voltage configuration by either changing the motor winding connections or the taps on the pole top transformers series/parallel for delta 460 or 230.
With a grounded 480/277 they work fine. Ive never seen a 480V motor before, always 230/460.
I haven't seen a 460V delta service either for that matter. Most likely because the power company would rather keep 240/480 transformers in stock because delta 230V 3PH is not offered here anymore. The 240's are used everywhere for single phase setups.
