Most likely they are being paid a day rate (flat rate regardless of the number of sex acts), an extra sex act was negociated prior to filming, or (most likely) the girl really enjoys sex and just wasn't finished yet.

One of the best and truest posts I have seen on XPT was someone asking how you get a porn starlet to suck your dick. The answer was "pull it out" since most are "trained" to fall on a dick whenever they see one.
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I'd rather be ignorant than stupid. Ignorance implies a lack of knowledge which is easily correctable through education. Stupidity implies an inability to learn. Therefore; ignorance is temporary, stupidity is forever!