The Japan average is also bought down a lot by one network (NTT DOCOMO). As well as the usual factors age of the network could also be an issue here. Both SoftBank (16.2 Mb/s), KDDI (14.8 Mb/s) are under 1 year old, presumably under capacity waiting for people to upgrade handsets and contracts. DOCOMO (5.5 Mb/s) on the other hand launched in 2010. You see a similar effect in the US, where AT&T's network is regularly beating the older, more congested Verizon. We expect this to even out over time.