He/she will sign it even if they are not fit for duty.

Maybe, prove it.

So you just told us that you maybe sign the fit for duty statement even when you aren't. That's comforting.

Sure, I sign it based on whether I feel I'm fatigued or not. It's my interpretation, I'm not being administered a cog test.

Acclimatization - needs to be addressed.
A East Coast SCR pilot can be assigned to a DHD at 12:00 into the SCR, DHD to London remain within the Table-B limits, receive 11:00 rest and fly the next FDP upto the full FDP limit in Table-B/C, while a Line pilot assigned to actually fly the two FDP's used to transport the first FCM, would need to apply the 0:30 penalty on the second FDP ... this is inconsistent and unrealistic ... who is more tired the line pilot on duty for 10:00 or the SCR on duty for 22:00?

Rescheduled FDP start time also needs to be addressed.
Technically, not even in the reg, it is just because of interpretation by the FAA, just write the regulation. The FAA could learn from the EASA FTL regs on this subject.

Disruptive Schedules (Day to Night and visa versa)- was partially in the original NPRM of the reg for SCR. FAA could learn from EASA and CASA on this topic.

117.23 - Cumulative FTL & FDP , again language does not support the FAA's interpretation (which I think is correct), so a loophole can be found and brought to suit in court. They can close it up by changing the language to:
No certificate holder may schedule and no flightcrew member may accept an assignment or continue an assigned flight duty period

For you to get to 18 hrs you would have to "extend" your FDP+RAP. There is no provision for that.

You can only extend an FDP. The longest FDP in the table is 14 hrs. If your RAP started at 0800, and you were assigned two legs with a showtime of 1000, you then start an FDP that is limited to 14 hours + 2 hr extension. This would mean 1000 show would allow you to go to 0200. However, you can't extend your FDP to a point that your RAP+FDP exceeds total limit of 16 hrs which means you must be blocked in at 0000 (0800 RAP start+ 16 hrs). An extension to your assigned FDP at this point would take you past your FDP+RAP limit.

Let's say instead that your RAP started at 0400. FDP+RAP limit is 14 hrs (Must be blocked in at 1800). You could start an FDP with a show of 5:30, (12 hr FDP limit takes you to 17:30). That means you can extend 30 mins and still be legal.

There are two concurrent time limits being run, your FDP+RAP limit and your FDP limit - they don't combine into one. Better training of this would save a lot of heartache (and ASAPs!)

Oh also like other people have mentioned, if you are fatigued, don't fly. Think of a normal flight. If it was a flight plan requiring fuel, the FDP limit would be your destination, the extension would your alternate. It's all a plan for legality purposes. The fatigue call is like diverting to a better option because things have deteriorated enroute with your destination and listed alternate- it's a real time solution.

You shouldn't be going over 16 on reserve...
FAR 117(c)(3)
For an unaugmented operation, the total number of hours
a flightcrew member may spend in a flight duty period and a reserve
availability period may not exceed the lesser of the maximum applicable
flight duty period in Table B of this part plus 4 hours, or 16 hours, as
measured from the beginning of the reserve availability period

Ahhh, read the extension section again, you can extend 2 hours past the 16, and the company expects you to do it.

Disregard. - I now realize there was a clarification letter.

Now I'm fatigued.

If you truly believe that, why are you not filing ASAP reports every day until that belief changes?