Why are some fares defined and others not?

Due to Southern cancellations used Tulse Hill rather than my normal Streatham Hill, from Canary Wharf, via Canada Water. Asked for a refund after being charged £2.00 exiting Tulse Hill and TfL Oystercard operator told me this was only a goodwill gesture and would not longer happen again. Seems the fare is defined to Streatham Hill but not to Tulse Hill, so you get charged at the latter. Since I haven't gone outside my zones 2 and 3 I don't understand the charge.

Any seggestions?,
Martin J
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