The XP amounts are set fairly low so you can get this ALONGSIDE other commitments throughout the month, such as:
-Skill of the week
-Clan events
-Your other RS stuff (Such as training skills

)
Hence the XP is set at levels which you can acheive while still going about your normal day to day stuff and just dedicate anything from 30 mins a day to no lifing the skill.
Obviously we could set the limits very high (ie expecting people to get 99's etc.) but the point of this is to have EVERYONE work on their warring skills, not just a few people who want to raise their warring skills extremely fast no life and get attendance while the rest are put off by the thought of having to work particularily hard on something they may not like too much.
If you feel the XP's are low, aim for the top teir or even to get the most XP in WG?
~Evil