This is a question that keeps coming up.
Under FPTP everyone makes a single mark on the ballot and every vote counts the same. So when people think about AV allowing a number of preferences, and the final result being based on some 1st preferences, some 2nd and maybe some 3rd or even 4th, they question why a persons later preference should count the same as someone elses 1st preference.
The answer to this is quite simple - if a persons 2nd preference is being counted it is because their original 1st prefence has been knocked out. Their original 2nd preference is their 1st preference of the remaining candidates!
So when it comes to the count, you count the 1st preference among the remaining candidates.
To extend this, to make it clearer still, even under FPTP are your 'ideal' candidates even on the ballot? If no then the person you vote for is not your real first preference - just like AV you can only vote for the 'best of who is available'.