Why can I still charge my e-cigarette battery more on the home/usb charger after charging with the portable charging case?

The charging case typically charges a battery to around 95% or more of its total capacity to indicate it is fully charged.  Since the charging capacity of the battery case, compared to a powered plug, is less efficient, the case will determine when it is most efficient in the end of the charging process and stop it on its own automatically to converse power for future charges.  This only occurs when the battery is considered fully charged and if the e-cigarette battery is charged with the USB/home charger after, it will only charge the battery slightly, if at all.  The case is "smart" and ensures you get the most out of it!

Since direct power from the home charger will always be constant, it is a more efficient means of charging and therefore has no concerns about the charge and power remaining on its own.  This will be true across any portable charging cases like this.