Consider the following scenario:
Vehicle starts driving and house battery voltage quickly comes up to 13.8v where the alternator wants it. The sun comes up and solar increases power until it is contributing 75w.
The alternator senses the voltage trying to rise above 13.8v and so eases back on current until the alternator is contributing only 50w and the solar is powering the chassis loads. It doesn't have enough power to overtake the demand and increase voltage above 13.8v; every time it tries the alternator cuts current to keep voltage stable at 13.8v.
If the solar had more than the chassis demand (125w) it would start bringing up system voltage. If it had 250w of output 125w would go to the chassis and 125w toward bringing the voltage up toward Vabs.