That is mentioned on Workbench Nostalgia as well. Might be somewhat of an older site, but it's always good for a readthrough.
Chris Young recalls that booting from some disks on Kickstart/Workbench 1.3 would show "Workbench Release 1.2" in the titlebar, but when booting from the Workbench 1.3.2 disks, it just showed "Workbench Release".
Thomas Kessler notes that this is due to a patch function called from C:LoadWB.
Ross Vumbaca adds more detail: "the Workbench version message in the titlebar is typically patched by the command that starts Workbench--LoadWB. If you run the LoadWB command from a Workbench 1.2 disk (i.e. Workbench 1.2), it causes the "Workbench Release 1.2" message to appear in the titlebar, regardless of whether Kickstart is 1.2 or 1.3. If the LoadWB command is run from a Workbench 1.3.2 disk and I believe from a Workbench 1.3 disk, it simply says "Workbench Release.". That's about the extent of it--simple a case of booting disks with the old LoadWB command (which causes the "1.2" message to appear). This sort of patching continues to this day... the latest Setpatch command for example, patches the titlebar in Kickstart 3.x to say "Amiga International" instead of "Commodore-Amiga".
Rodney Hester confirms that the titlebar "Workbench Release." appears in 1.3 as well as 1.3.2.
I think if you edit it in the ROM and then boot WB1.2 you'll see "Workbench release1.3 1.2" =P