Why is my period cycle on the shorter side (about 25 days)? I realize it's normal, but what's the bio-physio basis for the difference?

Hormone changes. As we ladies get older, the eggs that are left are not as healthy. The yolk sac isn't as strong, so instead of lasting 14 days after ovulation, it may only last 10 days, which makes the cycle 25 days instead of 28 days or so. You can still have a baby, but the shortening of the cycle is a reminder that time is running out for your fertility.