strange is a better word because you only get that efficiency when all the following conditions are met
1. wall of thorns placed
2. jet stream lvl 6 non flying creature
3. no ranged defence on creature
4. no imovability
5. no armour
not saying it still won't be good against a lower lvl creature with armour just it will have an average to below average efficiency.
from my excel.
for 1 wall and 1 push to mage (lvl 6) you have. 75% push chance gives
9.5/9 damage/mana efficiency (0 armour)
(9.5, 7.4, 5.2, 4.
/9 for armour (0,1,2,3+)
9.5/9 is approx as efficient as the current most efficent spell damage (flame blast 5.3/5 damage/mana efficiency)
but flame blast has better efficiency from 1+ armour.
now if you consider the cases for lower lvl monsters the efficiency drops drastically, most attack spells beat it in efficiency from lvl 5 down. (or 6 with 2 armour).
now consider you can push them twice.
that gives
1.46, 1.13, 0.8 DME for lvl 6 0,1,2 armour. 1.2 DME lvl 5 0 armour
1 DME lvl 4 0 armour
so if you can jet stream twice while wall is still up you do get enough efficiency to warrent its use on creatures lvl 4-6 and even lvl 6 creatures with 2 armour.
then finally if you can push more then twice jet stream is the most efficient and alternatively if you use it on lower lvl or more armour you have average to worst efficiency