How is the ANSWER of migrated thread get validated?

I hope i am wrong but... how is the answer of the migrated thread get validated? 

I took a few look on some migrated thread and it seem that ALL of them mark the first reply as the answer.... 

https://r1132100503382-eu1-3dswym.3dexperience.3ds.com/#community:yUw32GbYTEqKdgY7-jbZPg/iquestion:AodDfB1-RKipgiuBPZkmqg 

https://r1132100503382-eu1-3dswym.3dexperience.3ds.com/#community:yUw32GbYTEqKdgY7-jbZPg/iquestion:xLlaIfnGQESYU5H8BRgaAQ