Measuring from the hypothetical year 0, the Julian calendar has had 15 leap years that the Gregorian calendar didn't- 100, 200, 300, 500, 600, 700, 900, 1000, 1100, 1300, 1400, 1500, 1700, 1800, and 1900. But the Julian calendar is only 13 days off, putting the brief equivalence from March 1, 200 to February 28, 300.
Why was 300 chosen as the "first" skipped leap year, instead of 100?
I think a response I posted a couple of months ago should give the answer you're looking for: call back if there's something unclear or further questions.
The upshot is that the implementation of the Gregorian calendar in 1582 was designed to synch with the Julian calendar as it was in the year 325. That meant a correction of 10 days. Since then the Gregorian and Julian calendars have drifted further out of synch, so that the difference is now 13 days.