This is due to the default tab stop being set at .5" to accommodate a single digit. The number 10 extends further to the right so the text for that entry is moved to the next [1.0"] tab stop.
The Heading styles determine which headings are included in the TOC but how they display is determined by the TOC Styles. Edit the TOC Style to set a tab stop further to the right... If no more than 99 setting at no more than .75" should be sufficient but if it goes into 3 digits a higher tab stop may be needed.