Incohesive Russian military leads to significant casualties

 ISW:

...

Former Donetsk People’s Republic (DNR) Security Minister and current DNR military commander Aleksandr Khodakovsky claimed on November 5 that Russian friendly fire may have caused up to 60% of total Russian losses since the end of Russian offensive operations in Mariupol in mid-May.[14] Even if this statistic is exaggerated, the fact that a Russian commander is publicly speculating on such a damning indicator of Russian and proxy competency indicates the deep challenges Russian forces face. Friendly fire typically does account for a limited number of losses in war but ordinarily nowhere near 60% of total casualties, which demonstrates a lack of communication and command and control coordination between Russian forces. Russian and Ukrainian sources also reported that a Russian rotation returning to its base near Pavlivka, Donetsk Oblast on November 5 drove into a ditch constructed by army subcontractors without prior discussion or warning, further demonstrating a widespread lack of cross-training and coordination between Russian troops.[15] The frequent replacement of Russian military leaders, promotion of inexperienced soldiers, and cobbled-together Russian force composition including Russian contract soldiers, Russian mobilized soldiers, DNR and Luhansk People’s Republic (LNR) forces, and Wagner Group forces exacerbate the fragmented nature of the Russian chain of command and ineffectiveness of Russian forces and likely contributes to frequent friendly fire incidents.
...

The report also indicates many of the convicts recruited by the Wagner Group have also been killed.

Russia has suffered significant casualties; apparently, not all of them have been caused by Ukrainian forces.  It speaks to the ineffectiveness of Russian command and control, which was evident from the beginning of the war.  The Ukraine military has outperformed the Russians on many fronts and has gotten better as it received US and NATO weapons.

See, also:

Russia Sends Ill-Trained Draftees Into Combat Amid Losses, Analysts Say

 Russia is funneling newly drafted conscripts with little training to the front line in Ukraine’s east while mounting intensified but ineffective attacks and suffering heavy casualties, according to the Ukrainian military and Western analysts.

Grisly videos of Russian infantry in poorly prepared positions being struck by artillery have partly supported those assertions, as has reporting in Russian news media of mobilized soldiers telling relatives about high casualty rates. The videos, filmed by Ukrainian drones, have not been independently verified, and their exact locations could not be determined.

...