It seems the Russians are sending in much older tanks to and equipment in the first phase of invasion to exhaust Ukrainian weapons.(cannon fodder) The Ukrainians have the FGM-148 Javelin anti-tank weapon, which the Russians are oblivious scared of.
The Russians are taking loses - but these are young and less trained troops. Shocking tactics.
The tactics of the Ukrainians is (predictably) is not to engage the Russians in on open ground.