See Kendo UI v2019.1.220 Distributed Source Code module kendo.splitter.js _resize() method (Lines 285-240) 

1) The splitter _resize method iuses jQuery width() and height() methods to set pane sizes.

2) These methods get/set the clientHeight and clientWidth of the pane element rather than the offsetWidth and offsetHeight. 

3) If any pane has a border or padding, the last pane will extend beyond the limits of the splitter element.

4) Recommend using offsetWidth and offsetHeight to get/set all pane sizes.