I can not remember seeing anywhere that the EU Ref was advisory only.
Then you clearly weren't paying close enough attention my friend.
Parliament voted in the European Union Referendum Act 2015 for an advisory referendum, not a binding one. They could have gone for the latter, but they didn't.
Should the United Kingdom remain a member of the European Union or leave the European Union was the question and the two choices were pretty much...
Leave
Remain
The nature of the questions are irrelevant - the relevance is the nature of the referendum that has been enacted by parliament. In this case the referendum was advisory.
The last time the whole of the UK voted in a referendum - in 2011 on the method of voting in general elections we had this question:
At present, the UK uses the "first past the post" system to elect MPs to the House of Commons. Should the "alternative vote" system be used instead?
with 'Yes' or 'No' as options.
But that referendum was binding - the government/parliament were required to enact the outcome, unlike in the EU referendum.
Was indyref1 advisory? I cannot remember that either. I could just see Salmond telling the hordes of jubilant Saltire waving Scots "hang on folks, its only advisory and we will have another ballot later once we have a chat with Westminster"
Yes it was advisory - and for obvious reasons, in that the referendum was called by the Scottish parliament who have no authority to enact independence - that authority resides in Westminster.
There are also similarities between IndyRef and EURef (which are distinct to AV vs FPTP-Ref) which makes it necessary for the former to be advisory. Namely that in IndyRef and EURef there was no agreed settlement at the time of the referendum, so voters could only vote on a concept rather than an actual plan - so in IndyREF all sorts of critical issues, e.g. currency were up in the air. Likewise for the massive range of brexit options from Norway-like to the hardest of hard brexits. Hence the need for the referendum to advisory only and subject to negotiation and assessment of whether the actual deal agree is in the national interest.
The AV voting ref was different - the deal was clear and enactable without further negotiation/agreement. An alternative approach would have been to have a more speculative referendum on voting e.g. should we continue to use FPTP. If people voted no they were voting against FPTP, but not for anything specific, except not FPTP (a bit like EU ref and IndyREF). If that had been the vote it should have been advisory only. because the deal still needed to be struck, not just not FPTP, but what as an alliterative - pure PR, STV, AV, AVplus etc, etc.