I found a quote from this article
One thing you may not want to do with a press pot, especially a larger model, is use beans roasted less than 2 or 3 days before. What, am I crazy? Nope. There's a problem with ultra fresh beans and it is called "bloom". When beans are only a day or two off the roast, they contain heaps of Co2. Heaps of it, I tell you. That Co2 will translate into a massive bloom of brown suds on top of your press pot, possibly overflowing, but also making it easier for big particulate matter (your ground coffee) to hop and skip over the top of the filter portion when you first apply it. Bloom looks cool, but can make using a press pot more difficult.
The poor filtering part sounds questionable to me, so I googled it and found a few bloom-related comments on this blog post. It seems like some people start timing their soak time only after they see bloom.
This seems like the real reason to me. I know for sure that the air content in ground substances can vary significantly: one cup of flour can contain double the volume of another if one is loosely sifted and the other is compacted. The above quote is accurate about that in my opinion.
Apply this logic to coffee and in order to produce an accurate, general recipe for how long some compounds take to come out of coffee grounds, you need to factor out a variable between coffee grounds: how much air is in the grindings. Maybe the first small pour will saturate with coffee compounds, or drop in temperature quickly, reducing how much it can extract from the grounds. Once it squeezes out all the air, the rest of the water is poured in to do the real extraction work.