Graphics memory is allocated dynamically with Intel HD Graphics. Windows 7 and Windows 8 will have different requirements for graphics memory. Thus you will see different amounts reported by the Operating Systems. There is no bug here.
Could you right click on the desktop (Both Win8 and Win7), click Graphics Properties , if necessary click Advanced option, and when the Intel Graphics and Media Control Panel opens click the Options and Support tab.Then click the Information Center tab. Use this information to compare graphics memory. What do you see?