The reasoning for this is that defenses adjust more based on RBs and TEs than they do WRs (also easier to count). With 1 RB and 0 TEs defense will get their Nickel or Dime personnel in the game for better matchups.
00 personnel is 5 WR empty
10 personnel is 1 RB, 0 TE which means the offense has 4 receivers on the field.
20 is 2 RB, 0 TE
30 is 3 RB, 0 TE
11 is 1 RB/1 TE
12 is 1 RB/2 TE
13 is 1 RB/3 TE
14 is 1 RB/4 TE
21 is 2 RB/1 TE
22 is 2 RB/2 TE
23 is 2 RB/3 TE
etc.