Going even further back in history, in 2007 the attacking % were similar to those this year, i.e. clearly better than in 2011. It would be interesting to look at the stats of other big tournaments to see what is "normal", but I'm really too lazy to do this
And another thing I forgot: 4 years ago the tournament was in November, so in the middle of club season and the players had no time to prepare well for WC, while this year they could do so better. But then, in 2007 it was also in November and yet the players had higher %.
Overall average spiking ratios of all attackers in the tournaments:
2007: 41,6 %
2011: 39,6 %
2015: 41,3 %
As seen above, overall averages are quite close, which indicates that the high attacking ratios of top 10 spikers stem from individual skills of players instead of a trend in team level.
Another interesting issue in the last spikers list is that there was no MB included in the ranking (neither Kim Hee-Jin nor Khadambi Brackcides played as regular MBs)