This is a frequent question that affects mostly those of us who are not using the metric system. Graphisoft has been asked to improve this for we Imperial Unit users for more than 10 years...
Numeric fields in schedules are displayed extremely inconsistently.
Some field are formatted according to Options > Project Preferences > Calculation Units & Rules. This has been the most complained about feature - as many users want to see fractional inches, or feet and fractional inches - but only decimal feet or decimal inches are allowed there.
Other fields respond to the Dimension settings in Options > Project Preferences > Dimensions.
Still other fields - such as Frame Thickenss and Frame Width (scheduled as "Additional Parameters" for door types) - do not respond to any of those settings - and apparently have code within the objects themselves that determine the display format. To me, that's a bug. But, GS hasn't done anything about it. Ever.
IMHO, numeric fields need to consistently respond to formats. Somehow.
For Frame Thickness - one inconsistency that I just found while testing this for you is that the "Frame Thickness Inside" parameter DOES respond to the Calculation Units format. So, you can get it to display in decimal inches... and avoid the leading zero foot (0') text. But, of course, you then do not have fractions. So, a 3 ½" thickness will display as 3.5" (or however many decimal places you specify).
If you need fractional inches without a leading zero, the only workaround is to export the schedule and format it in Excel (etc). I gave a workaround 12 years ago or so that used an Excel formula to reformat decimal inches to fractional inches. Not sure if I know where that is after so many years though...
One of the forum moderators
AC 27 USA and earlier • macOS Ventura 13.6.9, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB