How is a rolltop closure meant to work on a pack?

Look at the picture of the Circuit on the ULA website; this is what the roll-top part should look like regardless of whether there's anything in the front mesh pocket.
http://www.ula-equipment.com/product_p/circuit.htm

Q: How does the top of the pack close? Is it easy to access the main body of the packbag?

A: The top of the pack closes like a drybag (also known as a rolltop) very similar to what river runners use to create a water resistant seal for their gear bags. Basically, the back of the collar is slightly taller than the front. Once you are packed, the collar can be rolled down upon itself to effectively create a barrier to water penetration. Once rolled, each side of the collar can be buckled to the sides of the pack or the two yellow buckles on the top can also be fastened to each other.
As for access, once the collar is rolled and fastened shut, you would need to unbuckle the compression straps and unroll the collar to access the inside of the pack. However, the rolltop only needs to be rolled shut when the weather dictates. If the sky is dry, the collar can simply be folded forward. Vertical compression can still be achieved by buckling the collar to the sides of the pack..

It's not a dry bag. You will still need to use a liner to keep things dry inside

Okay I get that but this is where I am confused with the 'roll top' function. Should it not be that both front AND back fabric be same length so that you get to hold them together and roll before clipping in? One is shorter than the other and this makes the rolling for water protection a bit of a misguided idea.

I am tinkering with the idea of a zip modification to be honest…..