I suspect that is due to the block and webs being the least supported, and thus the weakest and subject to the most flexing, at 2, 3, and 4.
Bearings are not evenly loaded all around. The bottom area is the most loaded as it takes the peak force of the combustion process, which can be well above 10,000 lbs peak on each rod with any significant HP. Check it out here in the Clevite-Mahle performance catalog; look at page 19, which leads into interesting info on why full grooved bearings are not always the best for the mains. (It is a trade-off versus better rod oiling.)
