I have pondered this concept on and off over the past few years. It's not like a serious concern or anything but I was curious if anyone else feels the same way...
The idea is this: running in open areas (next to the ocean, in a very wide canyon, or in the desert a la Forrest Gump) where the surrounding environment is many miles away from where you are causes you to run slower than you otherwise would in a "narrower" space (roads, surrounded trails, urban bike/run paths, etc.) for any given training day.
My theory is that this is due to the surrounding environment passing by your vision slower than a "narrower" space where you may observe a totally different landscape every minute. This relative observance difference tells your mind you are not travelling as quickly during your ocean-side run (where things are not rushing by you and the pier in the distance seems only a fraction closer than it did 10 minutes ago) as you are during your creekside bike path run. Because of this psychological response, you run lazier or without the same excitement and slow down.
Not real evidence I guess, but one of my running mates pointed this phenomenon out to me when we were discussing times of a nearby ocean half-marathon. The course is essentially flat as a pancake but there are almost no great times run there. Indeed neither he, I, nor anyone else we knew had a PR from this HM. I think only two have run sub 1:05 in its existence (and this is a well known older HM that draws a large crowd every year and is managed very well). My friend says he thinks the sole reason is because the horizon over the ocean is constantly in your peripheral (the course is mostly out-and-back) and the aforementioned phenomenon causes slower times.
Sorry if I may not have described everything clearly. Most likely just an outlandish excuse from the book of the has-beens! Let me know your thoughts!