The phraseology is all wrong.
The phrase "descend 5000ft" can very easily be misconstrued as some pilots will understand that as - reduce current altitude by 5000ft
ATC never ever issue instructions/clearances with relative altitudes. If any pilot is to understand this as a relative value, his instructor shall burn in IMC-hell for ages. Anyway, to avoid such suicides I use british "descend altitude 5000ft" phrase.
Only relative alt transmissions may be traffic informations ("oposite 12 o'clock, 1000ft above").