The graphical bars represent values between 0 and 100, but for some of the metrics, the "real" value can go beyond 100.
In terms of competition, 100 represents a difficulty level that means it would be near impossible to get a new page ranked for this keyword. For example, this might be the case if all the pages have PageRank 5 or higher, have 80+ authority levels and thousands of backlinks each.
This would equal a full bar and equals 100 on the competition level.
However, there can be keywords with even stronger competition. PageRank can be 6+ or 7+, we could be looking at tens and hundreds of thousands of links and authority values could be all close to 100.
In terms of displaying the data, it doesn't make much sense to differentiate between "impossible to rank for" and "even more impossible to rank for", especially since the value is theoretically open ended. There's an upper limit for what makes sense to even consider doing an SEO campaign for and anything beyond that falls into the same category of "not worth it".