On Finnish topographic maps, “easy to traverse” does not mean completely dry ground. It means that the mire is considered crossable on foot essentially anywhere and in any direction. “Difficult to traverse” is a much stronger warning: the Finnish mapping specification describes such a mire as extremely difficult, dangerous, or even impossible to cross.
In practice, an easy-to-traverse bog/mire in August may still be wet and springy. You can encounter soft peat, tussocks, shallow water and vegetation, and your footwear may get wet. However, you should normally be able to walk across it without specialized equipment such as bog shoes. Waterproof boots or footwear that handles getting wet are much more relevant.
A difficult-to-traverse mire can contain very soft or floating vegetation, deep mud, pools and channels of water, and areas where your foot or leg can sink substantially. I would treat the map marking as a reason to route around the area rather than assuming special footwear will make a direct crossing safe.
Conditions also vary with rainfall and the exact type of mire, so the map classification should not replace judgment on the ground. If an “easy” mire looks unsafe at a particular point, turn around or choose a firmer route.
For Lemmenjoki and Hammastunturi in mid-August, I would therefore interpret the map roughly as:
Easy mire = normally walkable, but expect wet feet and slower travel.
Difficult mire = avoid crossing unless you can identify a clearly safe route.
No bogshoes should normally be necessary for the areas classified as easy to traverse.
