Need help understanding frame rates and "Bob" de-interlacing

Hi all, I just have a question about de-interlacing and frame rates.

I often encode 1080i recordings of football (soccer) matches. The source frame rate is 25 fps (interlaced video), but lately I've been encoding to 50 fps to increase the smoothness of the video, because usually when I encode to 25fps it still looks okay, but it's not as smooth as when you watch it on the TV (while it's still interlaced).

Encoding to 50fps using the 'Bob' de-interlacing option, does actually increase the smoothness of the video, it definitely looks smoother, but the thing is, I've noticed that it kind of looks like it's in slow motion... not much, but just a tiny bit. Perhaps it's just me, but I wanted to know if you guys think it's worth encoding to 50fps progressive from a 25fps interlaced source. Does this seem a little strange to you more experienced encoders?

And can someone please explain what I'm doing when i'm encoding to 50fps from 25fps? I know I'm not actually adding any new frames, but why is it that the video actually looks smoother when I do this? And could it actually make the video look like it's slightly in slow motion?

Thanks in advance :)