I wondered this a few years ago, and Googled it fairly extensively. What I determined (no expertise in the field) is that eggs are remarkably similar to berries. Some are available nearly year-round. Others only "in-seaon". And, the "in-season" can vary quite a bit as well depending on such factors as elevation, local weather, available sunlight, etc. In other words, June berries (or eggs in this case) can show up much earlier in some areas, and much later in others.

I eat lots of eggs and berries :)