Verified answer

The biggest reason is the influence of the moon. The Earth orbits around the Earth-Moon barycenter, which then orbits around the sun. If the solar system was just the Earth and the sun, then maybe the orbit times would be constant.

Also the gravity from the other planets upsets things a bit; and general relativity gets into the act, causing the orbit to precess a bit.

The interval between perihelion passages is not the same every year because the Moon and planets pull the Earth out of position. In particular, because the Earth - Moon barycentre is 4400 km from the Earth's centre, a first quarter Moon around the time of perihelion can cause a delay of more than a day because the Moon is swinging the Earth around the barycentre towards the Sun whilst the Earth - Sun distance in the purely elliptical orbit has very little daily change.

You forgot about leap years. Also, there is technically 365.242199 or about 365 and 1/4 days in a year, making each year slightly longer. Every four years, there is a leap year that occurs, and a year is extended to approximately 365 days. That is the main reason why time between perihelions 31,556,926 seconds instead of 31,536,000 seconds. Hope I helped!

Maxwell

You forgot that the shape of the earth's elliptical orbit isn't the same from year to year. ;)

Because it isn't precise