Very odd MPG from one of my drivers

i own several trucks (car haulers) and have questions to owner operators or experienced drivers .
i noticed very odd mpg on all the trucks one driver drove . i have 5 trucks (he drove 3 of them and always have significant difference with other drivers) . he seems very experienced in the field we are working but mpg just killing my profit .
here is the example :
i have one particular truck running chicago il to houston tx (through dallas) , one driver drove this truck since september till february and his fuel spendings always been around $1800 for around 2500 miles , another driver drove 1 trip in this direction and did extra miles going to san antonio and still within 1800 range. the driver i am questioning took this truck for the first time and drove little bit over 2500 miles and fuel spending over $2400. all drivers drive 70 (per them)
same driver drove chicago to nj route 3 times and overspent $400 per trip compared to $1100 average (around 2000 miles).
this seem very odd to me and i don't want announce my thoughts about it .
truck is 2000 pete , c15 , 13 speed , high rail , all the maintainance is up to day , daily working truck
this is not show truck and not truck that falls apart on the road , just a working dog .
i appreciate honest thoughts and answers what might be happening

 

i got your point but :
drivers get fueled pretty much same place
1st driver fuels up after 721 miles , 2nd - 497 for the same amount ... we are talking highway driving only on cruise control

 

i did , checked out good