If three groups of three people walk in, you have 9 people. Still an odd number
You pull the two groups of 3 and fill 3 rows of two.
Exactly. You're looking to make an even number so you wouldn't group those together. You would make 6 with the first two groups, then add a group of 1, 3, 5, or 7 to the last group of 3 to make another even number.
STILL WONDERING WHY IT MAKES MORE SENSE?
HERE'S A VERY THOROUGH EXPLANATION
1)A single rider line can be an inefficient use of line space. Sometimes you'll have dozens and sometimes you'll have none. Odd and Even doesn't have this problem, especially if it splits near the end of the line. There will always be odd and even groups.
2)Whether you're actually a single rider, or a group that just doesn't care if they're together, those riders always make it to the ride quicker than the group line or will wind up waiting forever to get on because they're not needed. If a single rider walks in right behind you, he/she will end up 'cutting' you in line up ahead and making your wait longer or will wait longer than you because he/she isn't needed to fill in a space yet. With Odd and Even, the number of people in an odd group line vs the number of people in an even group line is likely going to be similar so people walking in behind you will probably get on the ride in around the same time as you. There wont be 20 single riders cutting you in line, maybe just two groups of 3 from the other line instead. In other words, the single rider line can add the Fastpass syndrome, making the standby wait longer. Odd and Even lines give similar wait times for everyone.
3)Single riders are useful in an attraction like Test track(6 persons per car) where, unless you get two groups of three, EVERY odd group will need to be filled in with single riders. Even a group of 4 adds up to 6 with single riders easier if a two person group isn't available. With the mine train you have a full train of riders, a high capacity ride vehicle leaving the boarding area much less often than a ride like test track. But, you still only need a single rider for that awkward open space when the groups don't add to the right number, something that happens only once per ride vehicle. With ride vehicles leaving less often, single riders aren't in high demand. They can be pulled from the odd line, or the extra time can be used to find groups that add up properly.
You can see how in the case of low capacity ride vehicles (Test Track), the single rider line can move very fast while in the case of a high capacity ride vehicles (Rock n' Roller Coaster) the single rider line might never move.
4) Odd and Even eliminates the need to leave that awkward odd number of seats to be filled. Say you first take 4 groups for every train, two from odd line and two from even line, to be fair. Perhaps this gives you 12, 14, or 16 people... then all you have to do is fill the EVEN number of seats, which is obviously easy to do with even groups and also easy to do with two or four odd groups. You can choose to fill the space from EITHER line, unlike with a single rider approach where you fill the train in with a bunch of single riders or with a perfectly numbered group thats 10 people away in line (and will take eons to make it up to the front with their full party or will lose one of their people or will get nasty comments and/or looks from all those people they're cutting in front of...)