I'm going to posit two possibilities here. The first is simple and fairly obvious - the posture you adopt on the bed is better for you as it appears to allow for better relaxation and seems removed of the danger of you falling asleep. Your attention also seems more established. As R. Barzell indicates, meditation is a tightrope walk between discomfort and pain on the one side and dullness and drowsiness on the other. You want to adopt a position where your body aids in concentration rather than detracts from it.
With that said, I personally think sitting that on the floor is actually doing you more good from a developmental standpoint. Meditation is not about blissing out, taking tea with the gods, while the lot of you float around on white poofy clouds. My own tradition (Rinzai Zen) likens meditation practice to ascending a mountain of swords; the bare skulls of those that have gone before litter the ground along the way. This is not to say that practice should be painful, ascetic, or in any way masochistic. The "goal" is still relaxed attention. What that metaphor does mean, however, is that as your wrestle with your body and mind, you will come face to face with situations that may be difficult and uncomfortable both physically and psychically. While the overwhelming temptation is to run, the only way to surmount these obstacles is to persist.
When we sit down on the cushion, establish our attention, and watch the breath, we are throwing open the door to our unconscious. Memories both good and bad, agitation that seems to have no discernible source, ill will and anger, sleepiness, doubt, and even deeper unconscious forms will arise unbidden. It's all part of the process. In fact, when these things do come about, while the content of experience is generally not important (and you really should do your best to put these experiences aside), their appearance is usually an indication that your meditation is going well.
I don't know what your goals are. If you are meditating for stress relief and relaxation - and there is nothing wrong with that! - I think your position on the bed is ideal. But if you are looking to climb the mountain of swords and add your own skull to the bone heap, get on the floor!