рассчитать частоту в R

Вот мои данные

> a
 [1] Male   Male   Female Male   Male   Male   Female Female Male   Male   Female Male   Male   Male  
[15] Female Female Female Male   Female Male   Female Male   Male   Female Male   Male   Female Male  
[29] Male   Male   Female Male   Male   Male   Female Female Male   Male   Male   Male   Male  
Levels:  Female Male

> b
[1] 0 1 0 1 0 0 0 0 1 1 1 1 0 1 0 0 0 1 0 0 1 0 0 0 1 1 1 0 0 0 0 1 1 0 0 1 0 1 1 1 1
Levels: 0 1

> table(a,b)
        b
a         0  1
          0  0
  Female 10  4
  Male   12 15

Я не знаю, почему результат table(a,b) имеет строку (0 0), мой ожидаемый результат выглядит следующим образом:

> table(a,b)
        b
a         0  1
  Female 10  4
  Male   12 15

Не могли бы вы сказать мне, почему это происходит и как это исправить, спасибо!

> dput(a)
structure(c(3L, 3L, 2L, 3L, 3L, 3L, 2L, 2L, 3L, 3L, 2L, 3L, 3L, 
3L, 2L, 2L, 2L, 3L, 2L, 3L, 2L, 3L, 3L, 2L, 3L, 3L, 2L, 3L, 3L, 
3L, 2L, 3L, 3L, 3L, 2L, 2L, 3L, 3L, 3L, 3L, 3L), .Label = c("", 
"Female", "Male"), class = "factor")

> dput(b)
structure(c(1L, 2L, 1L, 2L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 1L, 
2L, 1L, 1L, 1L, 2L, 1L, 1L, 2L, 1L, 1L, 1L, 2L, 2L, 2L, 1L, 1L, 
1L, 1L, 2L, 2L, 1L, 1L, 2L, 1L, 2L, 2L, 2L, 2L), .Label = c("0", 
"1"), class = "factor")