Jpeg change size

Everytime i export a jpeg from signlab 9.1 it changes the original size. I try and do a 18x24 and if i do 309 dpi it almost stays the same. If i change the dpi to anything else the size start to increase around 18.2x24.2 etc? Any idea? My signlab 7.1 exports in correctly.
 
Anyone try this and see if it changes on their signlab? Try exporting a 18x24 or any size jpeg, then import back in and see if the size changes.
 

iPrintStuff

Prints stuff
Haven’t used that program but I have a small printer that uses DPI for its sizing. It doesn’t actually edit the photos/pixels as it’s just a print software so when I change the dpi I’m literally changing the dots per inch so naturally that’s changing the size of the artwork. Could that be the case?
 
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