The answer can be easily summarized with this map. Look at how far north Ireland, England and Norway are. Yet they are far warmer than Greenland and Newfoundland! The reason is that warm water and weather flows from the Gulf of Mexico to Norway, keeping those places, that are near the sea, relatively warm. (Palm trees can grow in Ireland. Surprised me when first visited). Greenland and Newfoundland, on the other hand, receive cold water from the Arctic; the Greenland and Labrador current brings cold water and cold weather to these places.
Note that Siberia is too far from the Atlantic to be warmed by the gulf stream.
Something similar happens in the North Pacific. The Oyashio current flows from 'the north' to Vladivostock, keeping it cold. The Kuroshio Current, on the other hand, keeps Japan, Korea, and the Yellow Sea warm. (Port Arthur was located near Dalian in the Yellow Sea, so it was kept warm too). (Edit: I just read that it the Siberian High is more important that the Oyahsio current for Vladivostock winter. That Japan blocks the Kuroshio is important too.)
I think historically, Russia had a long term habit of taking land from China during its colonial expansion. Vladivostok was aquired from the Qing Dynasty in the Amur Acquistion in 1860, likewise in the region around Irkutsk in 1727. Russia had significant claims in Northeast China, for example in Harbin. It wasn't until after meetings between Stalin and Mao that Russia fully relinquished its colonial rights to Port Arthur; and late in Gorbachev's time, just before the USSR collapse, to resolve border disputes between Russia and China.
As this is a history not geology SE, I want to emphasize the source material that I quoted above. It is transcripts of the conversations between Mao and Stalin, where they speak frankly about the withdrawal of Russia troops from China and how to stop the US from invading. A second conversation is here. These documents are part of the Wilson Center Digital Archives that cover many interesting topics from the cold war.
Another note: I am uncertain if Vladivostok is ice-blocked in these modern days of Global warming, but I think in the late 1800's that the harbor was a giant salty ice cube.