In this picture you may notice the peak is a touch longer than the floor of the structure. It is not dimensionally perfect until the floor is anchored with 2X2 paving stones or equivalent. Doing this ensures the perfectly flat floor sets perfectly placed folds.

This is only an aesthetic issue.

The flush fitting end wall fits cleanly and smoothly to the tunnel.

Note that a flat surface under the front end wall is key to that tight fit. If installing on soft grass or uneven gravel, a 10' long 2X6 under the front wall edge provides all the surface you need.