Where are you seeing that the XT is different? Both specs sheets say it's a "5.25-inch woven composite low-frequency transducer".
https://pae-web.presonusmusic.com/downloads/products/pdf/Eris_E5XT_Studio_Monitor_techsheet.pdf
https://pae-web.presonusmusic.com/downloads/products/pdf/Eris_E5_Studio_Monitor_techsheet.pdf