Short statement:

Do not use TxMbps as a metric to aim your AREDN device's antenna.

Long statement:

When aiming an AREDN device's antenna, maximum SNR at each end of a link is the best metric to use.
TxMbps is a poor metric to use to aim an AREDN antenna, because
an AREDN device is always attempting random modulation coding schemes (MCS).
The '/mesh' page (mesh status) is updated about every 13 seconds.
TxMbps is calculated from MCS and retried packets.
It is common for the TxMbps to change when the '/status' page refreshes.

It is recommended to configure your link for maximum SNR and let the device choose the MCS.

A tool to aid in the aiming of an AREDN antenna is 'built-in' the firmware.
Click the 'Charts' button on the '/status' screen.
A visual and (optional audio) indication of signal strength is then displayed.
Once a link is achieved, you may also use this tool at the destination node.

73, Chuck