Hazards of 2014 not really being planned. Putin didn't think that not signing the EU–Ukraine Association Agreement would generate enough backlash that Yanukovych could and would be ousted. When that surprise happened, Putin took advantage of the chaos to secure Crimea, but I'm guessing he stopped there as stopping at Crimea's border would make it easier to defend and outside of Crimea he probably didn't feel he was guaranteed enough local support to secure annexation without real problems from the local population. Or maybe he just didn't have enough troops available at the time and he picked supporting the east over trying to make something more happen in the south. Likely no one involved in agriculture was involved in the decision making, or he'd have known he needed to at least push far enough into Kherson to secure river access or Ukraine could simply cut a vital water supply of Crimea's.
Oh yeah, Ukraine's have heavier losses than they'll admit to as well. The thing is that even 400 a day for a week is "just" 2800 casualties. That'd be fairly in line with the 2,000-4,000 Ukrainian KIA that the US was estimating
back around March 10th. Yes that's relatively high. The problem for Russia is that its KIA were being estimated by the US to be in the range of 3,000-10,000 as of
around March 18th, with 7,000 being one of the more broadly circulated estimates. It's a bad sign for Russia if they probably aren't doing better than 1:1 in losses with Ukraine and could be going as high as 3:1 losses. It's not really sustainable. Ukraine has a general mobilization going and is ready to throw conscripts(hopefully once they've been trained) at Russia, while Russia doesn't have the same. So if that ratio keeps up for too long, Putin runs out of available troops before Ukraine does, and he'll have to either leave rear areas or less crucial borders under defended, or expend the political capital at home to send in his own conscripts and maybe draft more, or break out the chemical or nuclear weapons to start reversing that ratio in his favor. None of which are going to be particularly palpable options for him.
To be fair, if Russia can secure it's objectives and revert to a defensive posture while eliminating internal resistance in the conquered areas, they'll likely start doing much better in losses as it becomes Ukraine's job to go on the offensive and recapture territory. Still this is going to be very bloody all around, and if Putin and his planners had known that ahead of time, they really should have come up with a better plan.