Deleting And Inserting Columns - Reached Its Limit

lets say i have 200 used columns. if i delete 150 of them and then (without saving the file) try to add 100 more i get an excel has reached its limit error. theoretically 200-150+100 = 150 columns, which should be well within the 258 column limit. but excel still gives me the error )its like its calculating 200+100=300 > 258). HOWEVER, if i save the file after deleting, i dont get any errors at all. is this a "feature" of excel and if so is there anyway of circumventing it? because i dont want to save the file prior to adding the columns (i do the whole thing in vba). actually i dont want to save the file at all unless the user click on the save button or choose save/save as from the menu.