Data disappears

Good day<br> <br> I ended last month with just under 618MB of data left on my account.<br> 500MB of that was allocated in beginning of November and the rest was from previous month<br> Now I know the rest should fall away but today with this new months 500MB added my account only has 688MB of data.<br> Why did the 500MB packages data that was added in November disappear first?<br> So are you saying that adding extra large data package is a waste since if you add bigger then your allocated monthly data gets used up first?<br> <br> So it is better to add as small a data package as possible?