It is not normal. Should be a fairly even gap. I'm assuming you're referring to a LCI above floor slide, which is like the one I've had problems with. Sometimes, but not all the time, there is a stop mounted on one or both of the slide rails. It secures with an Allen screw and is just a small tab. I had them add one to mine as it had none and pulled in too tight. Might check and see if you have one. If so, easy to adjust. It is normal for the top to tag first, but the bottom should pull up snug too. I can post pictures of the tab of you'd like.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.