If any devices are added between the 1st and the 15th of any month, those activated devices will be added to the following month's bill.
Any device added after the 15th of each month will just begin being billed on the following month. In other words, you WONT be charged for the month you added them if it is added AFTER the 15th on the next monthly statement.
You currently have 7 devices as of 4/1, and add 3 more on 4/6 totaling 10 devices. Because these were added before the 15th, you will see them added on 5/1's bill, including full amount and carried over onto the following month's bill. You will also see 10 license fees at $75 per device for the new month.