Nothingness: It is possible precisely because the games are chosen randomly and independently. With 2 sections of 5 players, there are 20 games, each of which is equally likely to be of any given type, regardless of how the other 19 games are assigned.

If you tossed 2 coins, would you be surprised if they came up both heads or both tails? I hope not; they're as likely to be the same as different. What if you tossed 20 coins and they all came up the same? Of course that's much less likely, but it's still possible; in fact you should expect it to happen, on average, once in every set of 524288 sets of 20 tosses.

What if, instead of tossing 20 coins, you take 20 hats, put into each hat 11 cards marked 1 to 11, thoroughly mix the cards in each hat, and draw one card from each hat? Will you be surprised if you don't draw any cards marked 2, 5, or 9? Maybe such an outcome isn't very likely, but certainly it's possible; the only alternative would be for the cards in each hat to be influenced somehow by the cards in the other hats.

Of course that last example is equivalent to how the games in your tournament were assigned. The following table, if I've crunched the numbers correctly, shows the approximate probability of exactly n game types being represented when 20 games are assigned in a random tournament with 11 possible game types.

 n   Prob.
--------------------------------
 1   0.0000000000000000000163508
 2   0.0000000000000857251
 3   0.000000000854404
 4   0.000000532498
 5   0.0000617406
 6   0.00212913
 7   0.0275495
 8   0.150025
 9   0.356337
10   0.351113
11   0.112784

So, if you create many such tournaments, you should expect to see 9 or 10 game types in the first round most of the time. 15% of the time you'll get only 8 types, and all 11 types will be played only 11% of the time.