"The Budweiser Method For Rating Women" joke

These three guys are in a bar, having a few beers, and checking out the babes as they enter the establishment. One walks in, rather attractive, and they "discuss" her "rating," which is on a 1 to 10 scale.
One says, "I'd give her a 7. She's really quite pretty." Another agrees, and so does the third.
The bartender, while bringing a new round of drinks to their table, overhears their rating of the young lass. He checks her out himself and says, "Nah, I'd only give her a 3."
"A 3? How can you give her a 3?" says one of the three guys at the table. "She's a real pretty girl."
The bartender, walking away, says, "Well, I use the Budweiser method for rating women."
The guys look at each other, figure the bartender has lousy taste in women, and go back to their ratings. Moments later, another young lady, prettier than the last, walks into the bar, and they confer between themselves and decide she deserves a 9.
However, the bartender, wiping off the table nearest to theirs, again overhears their rating of the gal. He checks her out himself and tells the fellows that he'd only give her a 5.
"A 5? How can you give her just a 5? She's absolutely gorgeous!"
The bartender casually replies that he uses the Budweiser method for rating women.
"The Budweiser method?" they puzzle, as the bartender returns to his post behind the bar. They are quite confused.
Three, maybe four minutes pass by, and then a stunning blonde, 5'11" goddess walks into the bar. Long luscious legs, sexy shape. Truly a work of flawless perfection. Without hesitation, the three "judges" at the table determine that this young sultress is, without any doubt, a 10.
However, carrying a case of beer past them to restock the supply behind the bar, the bartender once more overhears their rating of the girl. He glances studiously at her, and says that the best, the very best that he could give her, would be a 7.
"A 7 ?!? How in the world could you give her just a mere 7? She's friggin' gorgeous!"
"Well," says the bartender again, "I use the Budweiser method for rating women."
"Budweiser!" says one of the guys, exasperated. "What in the hell is this 'Budweiser method' for rating women?"
"Well, says the bartender, "the Budweiser method for rating women is the number of Clydesdales it would take to pull me off her."