Please explain the purpose of the resistors in this scenario
A traditional analog video signal is 1V pk-pk into 75 ohms, with black level at about 0.3V, so the sync pulses run from 0 to 0.3V and the video signal from 0.3 to 1.0V.

The MCU outputs will each provide 5Vpk-pk, so the two resistors are effectively in parallel (319 ohms). The output voltage is then 5V * 75/(319+75) = 0.95V. 
Not too far off.