The ratings process is not accurate enough to differentiate between round SSAs that are less than about 2 shots apart. In other words, we don't know if an SSA of 56 in the morning and 54 in the afternoon on the same layout among the same pool of players is due to more wind in the morning or just normal stats variance. As a player you might feel the difference but we have to go with the known stats accuracy and average the numbers.
Regarding tee time rounds, I'd prefer to just never provide ratings at all due to the issues you bring up regarding differences from morning to evening. But players want them so we do what we can and sometimes see situations where split round SSAs are generated. Note that the ball golfers are affected just as much with unbalanced weather factors. Their world rankings depend on finish position which can be significantly affected by their draw and whether they get the wind, rain, soft greens, etc.
Rater of the tossed arc.