The length of dashes depends on the preferences or the care of the type designer who created the font. In your example the difference is barely visible.
Here’s what the dashes are named according to the Unicode standard:
U+2015 Horizontal Bar
The en-dash usually has the same width as the letter n, and the em-dash has the same width as the letter m, so it is slightly longer than the en-dash.
The en-dash is used to replace “to” (“in the years 1955–2011”) and it can also replace commas to indicate a break of thought – such as here – where the sentence continues after the thought.
In US-American typography it is common to use the em-dash to indicate a break of thought—like this without spaces around—but I haven’t seen this in British typography. A little more information can be found here: http://www.typogui.de/index.html#dashes