Converting project triggers Engine recompiling, why?

I build a 4.11 Editor using, say, computer 1; I opened my 4.10 game project with it, the message windows wants a conversion, so I used “More Options > Convert in Place”, which cost a couple of minutes and it’s done.

When I download the editor into computer 2 and try to redo the above steps, this time the conversion triggers an Engine rebuild, also it fails, the command it’s calling is Running E:/XXX/Engine/Binaries/DotNET/UnrealBuildTool.exe Planets Development Win64 -project="E:/XXX/MyGame.uproject" -editorrecompile -progress -noubtmakefiles -NoHotReloadFromIDE -2015. There’s a -editorrecompile parameter so no wonder the Engine recompiles, but why?

My question is why no recompile in computer 1 but in computer 2? Thanks