It could be low quality coal (lots of aggregate and non-combustables). It could be an indication of softer coal which may be of decent quality (low aggregate, lwo ash content, good BTUs, etc.), but it simply burns faster. Could also be an indication of coal size being on the larger side. Of course go too large and the benefits of increased air movement through the coal bed are outweighed by the reduced surface area for the burn and thus will result in longer times to completely burn and lower MAX settings being needed. Coal size can also affect how well it feeds from the hopper or packs as it is moved down the grate.
Could also be a peculiarity of the specific stoker you have. Differences among the stokers of the same make and model due to tolerances in cuts, bends, welds, etc. can take away (or add) as little as 1/16" or 1/32" of travel compared to the "ideal" for the stoker mechanism and it can result in needing a MAX set at several points higher or lower than normal. Differences from motor to motor may result in a couple of percent variation in RPMs, also affecting optimal MAX setting.
Lots of things can be contributing. Couldn't say based only on a higher than typical MAX that it is specifically the coal quality that is to blame. To make that kind of determination you'd need to burn coal from at least a few different sources (or even different lots from the same source) and see how it affects your MAX setting.