Your source is;
"T-90". btvt.narod.ru. Retrieved April 11, 2014.
"Модернизированный танк Т-90С "Тагил" во всей красе". Retrieved 15 November 2014.
John Pike. "T-90". Retrieved 15 November 2014.
However last two have same figure as global security, and BTVT is not reliable source for Russian armour protection, although it does have a lot of information about Russian armors. For example, BTVT claims that the protection of M1A1 abrams is 400~450mm KE, but in reality it was 600mm according to Steven Zaloga and the Soviet Intelligence.