I might be wrong, but i guess the box takes the lowest speed limit and simply applies that. If you get a 220kts restriction by ATC and enter that as the below FL100 restriction it will follow that speed (with the limits quoted by skyjob), but you might get a scratchpad message UNABLE YYY KTS AT XXXXX to advise you that another speed limit cannot be met.
And i have to say, i often wondered about speed problems during arrival as well. I do understand the logic behind the 240 instead of 250, however other aircraft types (the bus for example) use 250 and create a 10kts difference which might become a problem if no speed control is applied by ATC.