The numbers are random and non-sequential, i.e. the existence of SEAL Team 6 doesn't imply the existence of 5 other SEAL teams. Equipment serial numbers are the same.
If you did assign numbers sequentially, enemy forces can get a guess as to your numbers based on the serial/unit numbers of captured equipment and soldiers
Hey, this is where my ability to remember stupid shit I don't want to comes into play!
SEAL Team 6 DOES imply that there are 5 other SEAL teams... if you don't know any better. It's literally the reason it was called team 6 and not 3.
At the time, there were two SEAL Teams, SEAL Team ONE and SEAL Team TWO. Marcinko named the unit SEAL Team Six in order to confuse Soviet intelligence as to the number of actual SEAL teams in existence.