For Grandma’s I decided to wear my 305 on my right wrist and my 405 on the left. I started each one just as crossed the start line. According to Training Center and Ascent (log program for the Mac), the 305 was started 2 seconds after the 405. 7:35:21 AM and 7:35:19 AM respectively.
I did not do mile splits but did do a split at the half. The 305 was 13.1 in 2:46:18 and the 405 had a 13.1 mile split of 2:44:56 (2 seconds off reported actual race time), What is really interesting is that both watches had my finishing time within 2 seconds, the same as the start, the 305 on the right wrist had 26.37 miles while the 405 had 26.22 miles. This is a difference of 0.15 miles.
The 405 had a total ascent of 3,285 feet and a decent of 3,395 feet.
The 305 had a total ascent of 3,056 feet and a decent of 3,162 feet.
Average and Max/Min heart rates were the same on both watches.
I could assume the course was longer on the right side a runner yet more hills on the left?
I could assume that the 405 is more accurate than the older 305?
I could assume that the reception on the left was better than the right?
Although, I will just assume it is science, and science will never be perfect… No matter how much we try or expect perfection.