The 7-Segment LEDs require 7V, but the i2c driver has only a max.
So after the I2C output you put a transistor that will handle the voltage you want.
Looks like you are uncleare about what the 7V actually means, it is not the voltage you need to supply, it is the forward voltage drop of the seriese LEDs so you must supply that voltage pulse the voltage dropped across your current limiting resistor.