Nobody here can really answer that question, but you can since you're using a desktop version of TurboTax.
Go into Forms mode, (icon in upper right hand corner), open one of your Forms 1040ES, put your cursor on the dollar amount in that form, right click and select QuickZoom. The "Estimated Tax Payment Options" worksheet will show up and you can see how that figure was determined. I'd guess that the difference between last year's tax liability and the estimated quarterly payments might come down to the expected 2019 withholding, but of course I can't be sure.
Tom Young
