Indeed, it's most frustrating for those of us who use it for the wider whole. The concept in this narrower sense was originally Russian, seeking to emphasise the cultural distinctiveness of the Tsarist and later Soviet space in relation to western & central Europe - some seeking in it a "greater Russian" identity, others a non-nationalist fusion of European and Asiatic elements.

Western usage seems to derive from post-Soviet scholars and political commentators for whom the Soviet-era concept offered a more convenient label than "former USSR". The less objectionable "northern Eurasia" enjoyed a brief vogue but was apparently too long for those who popularise these things.

And it gets even messier: the journal Soviet Studies became Europe-Asia Studies, while its peer Eurasian Studies covers a distinct though overlapping area "from the Balkans to Central Asia and Iran". I'm sticking with the original meaning.