Slip on = increased fuel consumption?

[ninot7]

Love my Blackwidow slip on only (stick headers). Fitted at around 10 k miles, now at 12+k miles. 
cant help notice that am pouring more fuel into my tanks (World Raid) at refuels. 
i normally fuel up at around 280miles and it used to take around 18-19 litres to full. 
i am now putting 21 Litres plus consecutively. 
 

question is, would a slip on like blackwidow increase fuel consumption? 
it doesnt bother me but just curious. 
thanks

[Hollybrook]

If it sounds better and you end up running the bike harder, that would be no surprise.  

[winddown]

I searched for ages for "stick headers" thinking some new slang had slipped past me.