One thing that could have an effect is the time of year you are born because it seasonal changes. A spring baby will be exposed to more sunlight than a fall baby in the first 6 months of life outside the womb, and the opposite for the second 6 months.

What that actually affects other than maybe the strength of their immune system early on, I can only speculate about. But it would be a mechanism through which birth sign can be correlated to personality or health.

And if there is such a correlation, it would make sense that the signs and dates falling out of sync as the constellations process wouldn't have any effect on anything because the stars and planets don't matter as much as the seasons do.

Though it does make me wonder if there have been studies analyzing personality difference statistics based on birthdate. Though I think it would have to be based on full birthdate (year included) due to year to year weather variations, as well as location dependent due to geographical weather variations. Like if there is such a correlation, it's probably stronger in Canada than California.