Why is the windshield washer fluid tank so strange?

I think I might have posted on this before but cannot find it now.

The Prius washer fluid tank is strange. It has a cylindrical region that extends almost an inch below the top of the tank. So it isn't possible, or at least easy, to completely fill the tank. Pour in the top and once the fluid reaches the bottom of the cylinder it only fills the cylinder, leaving a huge void at the top of the tank around the cylinder. I keep forgetting about this, correctly estimate the volume needed to fill the whole tank and it overflows slightly because the whole tank isn't used.

Why is the tank like that??? All I can think of is that it is to prevent sloshing from blowing the cap off when it is completely full.

Can anybody think of a reason a small hole couldn't be drilled through that cylindrical area, so that the air could drain out of (most) of that void, allowing the tank to fill (almost) completely.

 

On mine when it is filled to the very top the water line on the side of the tank is about 1 to 1.5 cm lower than the water level in the inlet. It is a pretty large empty volume on top since it goes all the way to the outer edge of the tank, or nearly so.

Expansion region to handle freezing makes sense. I never really think about that where I live, as it only very rarely gets down to freezing, and even then, just for a few hours at night. Freezing this fluid would only be a problem if the tank was completely full, which it very rarely is. In fact, the stores in Los Angeles cannot sell 1X washer fluid with methanol in them (AQMD rule - they say that all that methanol in washer fluid was a significant source of VOC, contributing to smog). The bottles are marked something like "protects from freezing down to 32F", which is so stupid it hurts, but there it is. I put it right up there with "gluten free water" and the like.