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The argument to CALL_FUNCTION is overloaded to show both the number of positional arguments and keyword arguments (shifted by 8-bits):
>>> dis("foo(10, opt=True)")
 1 0 LOAD_NAME 0 (foo) 
 3 LOAD_CONST 0 (10) 
 6 LOAD_CONST 1 ('opt') 
 9 LOAD_CONST 2 (True) 
 12 CALL_FUNCTION 257 
 15 RETURN_VALUE 
It is not obvious that the 257 argument causes three stack arguments to be popped.
The disassembly should add a parenthetical to explain the composition:
>>> dis("foo(10, opt=True)")
 1 0 LOAD_NAME 0 (foo) 
 3 LOAD_CONST 0 (10) 
 6 LOAD_CONST 1 ('opt') 
 9 LOAD_CONST 2 (True) 
 12 CALL_FUNCTION 257 (1 positional, 1 keyword pair)
 15 RETURN_VALUE