Interestingly, changing the top margin on my own paper to match the 1/8 inch of Epson's top margin did not affect the outcome, so apparently it's some combination of the top and bottom margin (.556) at play.
Given all that, and the fact that it -still- was off by 1/16, I've decided that it's actually easier to keep all my custom paper sizes (with 0 borders all the way around) and just adjust the top margin in PrintTool.
I get an exact match, top and bottom, by adding 0.19 to the current top setting. That's really pretty easy, for the math adverse, since it's just adding 0.2, and subtracting 0.01.
So: if the top margin shown on center is, say 1.29, then adding 0.2 = 1.49, and taking out 0.01 yields 1.48.
That is, by the way, approximately 3/16 of an inch.
At least for me, running El Capitan, PrintTool, Epson's driver and sheet feeding with 0,0,0,0 margins, this technique produces exactly balanced top/bottom margins.
Finally, as to "why so fussy?" (in case any lurkers are wondering) it's this: my client specified 11x14 paper, and some of her images extended to within 1/2" of the edge when properly centered. When the centering was thrown off, it resulted in a print that could not be properly matted.
I've been using Epson printers since the 2000 and have never heard an explanation of what Epson is doing internally with its math regard print placement and size. Their manuals contain tables showing what appear to be arbitrary "adjustments" to paper size and image adjustment to print borderless, but no explanation. In fact, a few years back, I called Epson pro tech support and directly asked them. They had no explanation either: "Just do what it says on the chart. I don't know why."
As a retired programmer, I could (and have) guessed at a few explanations (their ALU; the dictates of stepper motors and so on.) But the mystery remains...
Thanks for your (usual) patience and courtesy, Roy.