I am getting surprising results as I move into the ancient era.

I play at slow speed, so I consistently get a dozen or so science finds in the neolithic era.  But sometimes when I create my first city and choose my first tech, it is zero turns for any of the initial choices -- presumably because I have been accruing science points from my finds in the neolithic.  Other times, I find I have nine turns to obtain any of those first tech choices.  This makes no sense to me.  I am not making any different choices.

Any idea what could explain this?  Or a place in the interface where I can keep track of what is going on?  I see no accounting for accrued science during the neolithic era, and only a progress bar once I choose a tech in the ancient era.