Russian here. First, it might seem to you that powers that be can simply fabricate whatever data they want, but this is not the case. The elections are presented as democratic, they are monitored to some extent, and this year monitoring was more efficient than in any previous elections, so fraud is non-trivial. The most typical method is ballot stuffing and it is detectable using statistical methods.
Second, you're overestimating the "political machine". There was no properly designed fraud with some super-intelligent planning at the top. It would require proper coordination and commitment on all levels, not to mention some real skill, while in a corrupt government lower levels always try to reap whatever they can for themselves, while pulling the wool over eyes of upper levels. That's how the whole thing eventually falls apart. What upper levels care most about? Local election results. So the lower levels clumsily try to fabricate them.
The fraud wasn't even that blatant. By different estimates, only 1/3 to 1/5 of the votes United Russia got were fraudulent.