The difference is that Sarah was already a gedolah before the 10 year count started. If she gave birth to Yitzchok at 90 and Yishmael was 13 years older than him Sarah was about 77 when hagar conceived which would have been at the end of the 10th year when Sarah gave Hagar to Avraham. If this is true then Sarah started counting at 67.
No, Sarai started counting at 65 when she entered Cannan. She gave Hagar to Avrum when she was 75 and Yishmael was born when she was 76 and had his Bris when she was 89.