Unfortunately you never know when a ticket inspector has that sort of personality...I see , a stickler.
And if you knew what my ticket had cost me over the last 40 years, and how emphatically the rules have repeatedly been brought to our attention in the recent past, you too would make absolutely sure you complied with every single rule (or even hint, if it seemed fair and accurate) to make sure you didn't get into trouble.
I was challenged (unfairly) about 30 years ago, luckily my absolutely accurate and justifiable story was eventually accepted and nothing unpleasant followed... other than the initial hauling up before the boss, but it put letters on my staff file and unfairly brought me to the attention of quite senior managers well above and away from my location.
If a cheap ticket is only valid on the specified train, how is it that being in the specified seat doesn't matter?