Lighter bike slower than a heavier bike, possible reasons? I have access to two mountain bikes, a lighter one (lapierre) and a Giant. 
The maintenance I do to both is really similar, just tires and oiling the chain every few weeks. I always though that the lapierre was harder to ride but the other day I did an actual test. Before I set off I checked the tires and oiled the chain and did the same distance, at the same time and  similar weather and the lapierre was much more work and did a slower top speed and average speed, can anyone suggest any reasons for this and how can I fix it?

I took a bunch of pictures of both bikes first the slower Lapierre then the two bikes and then the giant here
http://imgur.com/a/G1MST#7

Tyre pressure is for both bikes on the 30s

I think tyre size is 26" (not sure) tho, as you can probably see from the pictures, they are the same size

Edit: very similar tires (if anything the giant has more grip), suspension exists and set to as stiff as possible on both (the test was performed on road), 
As for different gearing I have no idea how to check 
rider position feels very similar
Will take pics today and these tests are done on roads (ie while commuting)

My measurement was done using strava but also I felt it was harder to ride and thought I might have been slower 

Cheers
If you're measuring via Strava, keep in mind there are a lot of factors that can play into your time.  Unless you're averaging many rides over time on each of those bikes, thing like wind, clothing, what you ate, etc, can all play in to the time you post to Strava.