If layer1 is alone in an ArcMap frame, the LegendInfo returns just that layer's info.
If layer1, layer2, layer3 are in the same frame together:
I ask for LegendInfo on layer1, I get info for layer1, layer2, layer3.
I ask for info on layer 2, and get info for layer1, layer2, layer3.
I ask for info on layer 3, and get info for layer1, layer2, layer3.
Because the "mapServiceInfo operationDidRetrieveLegendInfo" method has no idea which of the 3 layers I asked for, and they can be returned in any order, I don't know which to display.