It's also worth mentioning that the correlation between PBs and doing well at the olympic marathon is farily low. There are tons of Africans who have run sub 2:06 on fast courses around the world who were handily beat by Jared Ward who, at the time, had a PB of 2:13. That is what, two minutes slower than the Olympic Standard right now? And he got 6th!
Most major marathons are run at a time of the year when weather is optimized and the course is flat. The Olympic marathon is run in the summer in a place that might be hot or humid and the course might be slow.
When conditions are not ideal, it's a major equalizer. It would actually be dumb to make the standard too stringent. It's expected to be hot and humid in Tokyo. It will be hot and humid in Paris. Also tactical marathons are a different beast.
Seriously, look at seed times and how they actually placed. The correlation isn't terribly high.