One reason - medical.
I cant imagine, unless its dealine day, that both parties sign contracts without a medical. Hence not 100% done nor were contracts signed.
Would you buy a car from another party and sign the V5 log book without having a security check or HPI check etc?
Chadli was announced pre-medical.
The only good reason I can think of is Valencia wanting more time before they announce that they need a new striker.