No problem Ellie.
There's no question in the EEA (PR) online form which asks what your qualifying period is - so, unless you tell them either via a covering letter or via inserting a paragraph in the "Further Information" section, it's seems that they assume it's the most recent five years for which you've provided evidence of exercising treaty rights (so in your case, 2013-2018).
At this point - worst case, which isn't really a worst case, is that you'll just need to wait until 05/09/2019 to apply for citizenship. Waiting time may be longer then, but they may also not be. You can use these months to gather all the info you need for the application, do the Life in the UK test, do the English test - the whole process is quite expensive so you really don't want to make mistakes.
The HO Guidance Notes for the EEA (PR) application say:
"Please note that once the application is concluded, we are unable to revise the date you are deemed
to have acquired permanent residence without a fresh application being made. It’s therefore important
that you provide sufficient evidence with your application for the whole period that you want us to take
into account."
[p.18 of
https://assets.publishing.service.gov.u ... tes-v3.pdf]
So although the guidance says the above, it may be possible to get them to change the date they deemed you to have acquired PR if you write to them stating why you think you should have an earlier date. So assuming the HO got it wrong, you should write a letter to the same address you sent the original application to, addressing it to the case officer/team indicate in your PR decision letter. You should politely explain why the date should be different and ask that they issue a new letter.
If the above is unsuccessful, you could submit a new application - ensuring you ONLY send evidence of exercising treaty rights for 2010-2015 and that for 2016-2018 you ONLY send evidence of continued residence. And ensuring you send a covering letter explaining everything and what you believe the qualifying period is.
For now I would just enjoy the moment! I'm sure it's taken a lot of work/stress to gather all the info for the PR application, so enjoy the fact you've been successful on your PR.