Fuel Cost Changing

first of all, i dont know if it was a bug, or maybe i miscalculate something here...

i play theme game july.
i was setting up a route CGK-PLM 261 miles with ATR 42-200.
at first, it gives me daily profit up to $24,000 with fuel cost as low as $304 in each flight.
and then i set another route and leave it for about an hour.
when i see it back in the Routes tab, i found all the routes flown by ATR 42-200 are giving a massive daily loss!

for example, CGK-PLM i have said above suddenly appear with -$56,000 profit!
and when i check what is went wrong, i found that the fuel cost is more than $15,000!

i thought maybe i miscalculate the ATR42-200 specifications.
but when i check, it still said: Hourly Cost: $1,230 and Fuel Flow: $368
which means, she mustn't be that expensive in using the fuel!

for my first aid, i immediately buy used F100 and change the aircraft flown by ATR42-200.
amazingly, another "weirdo" happen.

take CGK-PLM for example.
i have in about 35x weekly ATR42-200 who suddenly gives -$56,000.
and then i add 14x weekly F100, and the profit change to -$2,830.
?????
i mean, it was 49x weekly then, but it seems the loss is reduced.

and when i check the fuel cost, the ATR fuel cost suddenly back to $305!

my question is:
"is this common and usually happen?"

am i miscalculating something?
is there any possibilities of bug?

Please, i really need this help.
i've already order 10 new ATR 42-200, if actually this aircraft is unprofitable, i will stop the order immediately.
but if this sudden "increasing fuel cost" is only temporary and in fact only happen at the first week, i will be steady and wait for the normal fuel cost.

thank you...