The installation instructions for this double oven read:

"This oven requires a separate, grounded 4-wire, 240V (AC), 50 Amp service with its own circuit breaker. Installation must comply with National Electric Code... We strongly recommed connecting this oven to No. 8 gauge copper wire."

Here is the question: Doesn't a 50 Amp circuit require #6 gauge wire? Why are they recommending #8 gauge wire when, according to my knowledge, #8 is only used for 40 Amps or less?

Any clarification would be greatly appreciated! Thanks in advanced!

You are partially correct. It depends on the type of insulation and it's temperature rating. #8 Romex and UF cable is limited to 40 amps. USE and THWN are rated for 50 amps, THHW is rated for 55 amps. USE is often used for stove circuits.