A number of differences for 675LT...
1) they made over 1000 of these between coupes and spiders - only 500 Senna’s total are made.
2) it’s based on an existing car - so to most people it’s just a variant or special edition. Senna is completely unique.
3) it was sub-400K at release. Senna is nearly $1MM MSRP.
4) its a Super series car. Senna is an Ultimate series car
5) Senna purchase was invite only.
6) well, because it’s already appreciated - auction sold one for $1.45MM in January - half a mil more than msrp.
Probably more reasons, but that’s the highlights.