Why does Fedex not know the size of it's own flat rate boxes?

[pnw_studios]

I used FedEx 2 Day One Rate Small Box and it has a standard set of sizes. I was purchasing this shipping with the understanding that it was a flat rate (one RATE) which accounts for the size selected and any weight under 50-70 lbs. But now I'm being charged $4 after the fact because the size wasn't 4x4x4 or whatever the default was, but rather the larger size of the flat rate small box.

This seems extremely misleading. I remember this happening to me in the past and neither ebay nor Fedex would help.

[johnrj1226]

The flat rate for a FedEx 2Day One Rate shipment in a small box is $16.75, with a maximum weight of 50 lbs. The small box dimensions are 10-7/8" x 1-1/2" x 12-3/8" or 8-3/4" x 2-5/8" x 11-1/4". This pricing applies when using official FedEx One Rate packaging and meeting the weight and size requirements. 

• Cost: $16.75
• Service: FedEx 2Day
• Packaging: FedEx Small Box

• Maximum Weight: 50 lbs