You are wrong, there are only 3 penetration items, Youmu, LW and BC. Youmu sucks at penetration, leaving only LW and BC, BC is not a carry item since it lost it's AS for HP, leaving only LW.
The problem is the ghost stats. I don't know what effects this, enemy's item, a program bug, whatever, the problem is that sometimes having 300 armor or mr is equal to the base stat only, the same thing happens with movement speed. Remember when a bootless jungler could reach you when you had complete boots? Yeah, ghost stat. Same thing happens with Hecarim and Rammus, ghost stat on movement speed when they use the skill, the numbers go up, but your real speed remains.