I have a router ZyXel Keenetic. 300 Mbit/s, 2 Antennas, 2T2R, 2.4 GHz only.
Xperia Z1 connects only at 72 Mbit/s maximum. However, other devices, such as сhinese Zopo ZP300+ connected at 150 Mbit/s. I think that Sony limit connection speed somewhere, as often do notebook manufacturers to certify devices in areas where WiFi is limited certificate speed 72Mbit/s.
Xperia Z1 also uses 20 MHz, although the same ZP300+ calmly uses a 40 MHz.
I think Sony did so:
if you are using 2.4GHz connection is limited to 20MHz bandwidth and the speed of 72 Mbit / s. (maximum)
If you are using 5 GHz, the connection is 40MHz bandwidth and the speed is 150 Mbit / s. (maximum)