What Makes Tails Appear?

[Bee]

I have a few runs under the belt and one thing I've noticed is that the fusel oil doesn't appear proof-wise at the same time in different runs. One run it was at 35%, another at 46%. Why is that?
Using a pot.

[HDNB]

could be a few reasons. ferment paramters. type of mash. starting abv. passive reflux by cooler temps on the column. variable take off rate. depth in the pot.
I'm sure there is more possibilities, those are the ones that come to mind at the moment.

[Expat]

+1 to what HDNB said

Understand that fermentation is a chemically complex process which results in a solution of different compounds, all which will may react slightly differently depending on the exact conditions its processed. Many-Many-Many variables will result in small differences(usually) in the outcome.

E.g. if the PH of your ferment is out of band (for whatever reason) this may stress the yeast into producing a larger than average amount of other compounds which we call heads/tails. Without laboratory level detail its almost impossible to determine for sure the exact differences from run to run, but its not something you should worry about.

[Saltbush Bill]

And there is one good reason why no one should try making cuts by ABV.