N*k+N vs. N+1 calls for same generator expression

<redacted>$ python
Python 2.7.17 (default, Apr 15 2020, 17:20:14)
[GCC 7.5.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import random
>>> import string
>>> import cProfile
>>>
>>> def chrs(bits, len_string):
...     return ''.join(chr((bits >> (8*x)) & 0xff) for x in range(len_string))
...
>>>
>>> def bits_strings(num_strings, len_string):
...     return list(chrs(random.getrandbits(len_string*8), len_string) for x in range(num_strings))
...
>>> cProfile.run('bits_strings(1000, 1000)')
         2005004 function calls in 0.802 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.803    0.803 <stdin>:1(bits_strings)
     1000    0.001    0.000    0.799    0.001 <stdin>:1(chrs)
  1001000    0.608    0.000    0.699    0.000 <stdin>:2(<genexpr>)
        1    0.000    0.000    0.803    0.803 <string>:1(<module>)
  1000000    0.091    0.000    0.091    0.000 {chr}
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}
     1000    0.003    0.000    0.003    0.000 {method 'getrandbits' of '_random.Random' objects}
     1000    0.095    0.000    0.794    0.001 {method 'join' of 'str' objects}
     1001    0.004    0.000    0.004    0.000 {range}


>>> cProfile.run('bits_strings(1000, 1000)')
         2005004 function calls in 0.760 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.760    0.760 <stdin>:1(bits_strings)
     1000    0.001    0.000    0.757    0.001 <stdin>:1(chrs)
  1001000    0.579    0.000    0.663    0.000 <stdin>:2(<genexpr>)
        1    0.000    0.000    0.761    0.761 <string>:1(<module>)
  1000000    0.084    0.000    0.084    0.000 {chr}
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}
     1000    0.003    0.000    0.003    0.000 {method 'getrandbits' of '_random.Random' objects}
     1000    0.088    0.000    0.751    0.001 {method 'join' of 'str' objects}
     1001    0.004    0.000    0.004    0.000 {range}


>>> def chrs(bits, len_string):
...     return ''.join(chr((bits >> (8*x)) & 0xff) for x in range(len_string))
...
>>> def bits_strings(num_strings, len_string):
...     return list(chrs(random.getrandbits(len_string*8), len_string) for x in range(num_strings))
...
>>> cProfile.run('bits_strings(1000, 1000)')
         1005005 function calls in 0.181 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.751    0.751 <stdin>:1(bits_strings)
     1000    0.001    0.000    0.747    0.001 <stdin>:1(chrs)
     1001    0.001    0.000    0.751    0.001 <stdin>:2(<genexpr>)
        1    0.000    0.000    0.751    0.751 <string>:1(<module>)
  1000000    0.085    0.000    0.085    0.000 {chr}
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}
     1000    0.003    0.000    0.003    0.000 {method 'getrandbits' of '_random.Random' objects}
     1000    0.087    0.000    0.742    0.001 {method 'join' of 'str' objects}
     1001    0.004    0.000    0.004    0.000 {range}