activity display

Hello,
In the work input screen i've gto a sub project with 3 activities. Why do i get the first activity in grey and with a "-" button near it while it's just a simple activity with no sub activities link to it

in this exemple the sub project is 1- Lot unique and sub activities are Ananlyse du besoin, spécification and conception. why do i get Analyse du besoin in grey ?

all my activities got the field "parent activity" empty

here's a screen of my activities


also here's a screen of the field "parent activity" from the "specification" activity