Great observation. In modeling mode this is the intended behavior.
While that might initially surprise you, here's the rationale, and the benefit : modeling is often an exploratory process - if you don't know what's out there in your data, you use trial-and-error to look, manipulate, derive, etc. Not all of these exploratory steps turn out to be steps that you will want to keep.
To make exploration as safe and immediate as possible, Xcalar is designed to support movement in both directions, both forward and backward in your jouney. Furthermore, Xcalar also allows you to undo and redo instantly, so that you have greatest flexibility to change your mind as you go.
Behind thes scenes, when you take a step forward by changing something (modifying a field, deriving a value, etc), Xcalar preserves both versions of the table, pre-change and post-change. That way Xcalar can instantly revert to the pre-change state. Or, if you do choose to revert, you can change you mind again and jump back to the post-change state instantaneously.
Another great benefit is that Xcalar preserves the lineage (ie: the step-by-step history of your entire exploratory journey) so that you can see all of the steps, changes and versions that happened from start-to-finish. Xcalar presents this information graphically via the dataflow graph panel, which you can get to at any time via the dataflow graph icon in lower right corner of worksheet page.
Any time you want to inventory or delete older versions of tables, Xcalar will show you all your current tables in the System->Monitor menu choice on left edge of screen. You will see a menu-option which says Release Memory, with a trash can icon next to it.
Click on that icon and it will display the Drop Table dialogue which will list all of your existing tables - and show the size of each.
By the way there are three categories of tables that it will show:
a) Active (this is most recent version of the tables you are working with)
b) Hidden (if there are any, they are versions of tables which you have chosen to keep but hide from display to avoid clutter)
c) Temporary (these are prior versions of tables which have been saved so that you can instantly revert if you wish - you should delete this once you know you have no need to revert back to them)
Displaying these tables does not risk dropping them. If you do wish to drop any, you must click on them and then subsequently click on the Drop Tables button in the lower right).