"Why did my data usage increase while I was on vacation"
Answer: Because you powered off the modem. When you power off the modem you break the connection between the Modem and the Gateway. This means that the modem is unable to report the data you used between the last "sync cycle" and the powering down.
The best method is to:
Disconnect the Ethernet cable(s) between your devices and the Modem but leave the modem powered up.
If using the WiFi modem it is necessary to disable all WiFi functions (2.4 and 5,0 Ghz and all guest accounts),