A simpler example would make this easier to follow ..

    % proc x {t} {while {[t]} {incr x}}
      Command 1: "while {[t]} {incr x}"
        (0) jump1 +15       # pc 15
      Command 2: "incr x..."
        (2) startCommand +12 1      # next cmd at pc 14, 1 cmds start here
        (11) incrScalar1Imm %v1 +1  # var "x"
        (14) pop
      Command 3: "t..."
        (15) push1 0        # "t"
        (17) invokeStk1 1
        (19) nop
        (20) jumpTrue1 -18  # pc 2
        (22) push1 1        # ""
        (24) done

    % proc y {y} {while {$t} {incr x}}
      Command 1: "while {$t} {incr x}"
        (0) jump1 +6        # pc 6
      Command 2: "incr x..."
        (2) incrScalar1Imm %v1 +1   # var "x"
        (5) pop
        (6) loadScalar1 %v2         # var "t"
        (8) nop
        (9) jumpTrue1 -7    # pc 2
        (11) push1 0        # ""
        (13) done

Is this due to the optimisation at https://core.tcl.tk/tcl/artifact/49a6bfc698a0d1c0?ln=825 ?

It looks legitimate to me, since START_CMD only (?) exists for book-keeping
for the error path and interp limits.  Note that optimisation is skipped if
an interp limit exists.

The [info cmdcount] discrepancy is incorrect, but I understand [info cmdcount]
to be (necessarily) fragile in the face of BCC.  Probably the manual
should be updated to reflect what is said in https://wiki.tcl.tk/9813