Does USB supply a "constant" 5V or "up-to" 5V?
TL;DR just because it's connected to the USB does not mean 5V or even that it is powered from USB.

For versions 1 and 2 of USB The spec says 5V +/- 5%, so you should design to accept 4.75V to 5.25V. 

USB Version 3.something introduces the availability of increased voltages (up to 20V). but a device has to explicitly request increased voltage, else they just get the same Approximately 5V.

When I plug my phone into its charger via a power meter I see the voltage jump to 9V

My laptop's charger uses a USB C connector and can provide 5,9,15, or 20V (according to the label)

Now, your laptop's internal device may be designed for some voltage other than 5V. The USB data lines D+ and D- signal at only 3.3V.  So, 3.3V could instead be the  supply to the (touch-screen) digitizer. 

In general the USB host can't see the device's supply voltage, it only sees the data signals.
Some USB devices (eg: laser printers) do not use the USB VCC line for power.