There are many different historical numbering systems, based on how formations evolved in each country, and it could be argued that Uruguay’s is one of the most logical.
Uruguay won two of the first four World Cups, including the inaugural edition as hosts in 1930. Shirt numbering wasn’t in operation then, but when it did come into widespread usage, Uruguay were the same as the rest of the world in numbering left-to-right in the old 2-3-5 system:
Other countries transitioned from the 2-3-5 to four-man defences via the W-M, and that is where discrepancies in numbering arose, as different players would drop back in different countries, keeping their original number.
While the Uruguay team which won in 1950 in Brazil would have the front half of the W-M, they adapted differently in defence, moving directly from two men to four. As a result, it made sense for the right and left halves – numbers 4 and 6 respectively – to become wide defenders, outside the full-backs, 2 and 3. As in Argentina and Brazil, 5 remained as the midfield fulcrum.
Over time, 7 and 10 would retreat to midfield to join 5 and 8, but by and large it’s a numbering system which remained strong in Uruguay, as can be seen from this line-up, chosen at random, from a 1989 Copa America tie with Argentina: