A champagne cork needs to be under more compression because the bubbles are trying to drive the cork out of the bottle. If you look carefully, you'll even see it's not all one piece. The lump on the top gives you a place to grab to open it, and allows the wiring to "safety" the top.
If I understand you correctly, you mean that the shape changes after removing it?
What the splaying of the "shaft" of the mushroom means is that the cork has not been that long in the bottle. If it does not splay, it has been there longer.
Is it a good or a bad thing?
Well, 99% of all sparkling wine, and roughly 95% of all Champagne is meant to be drunk upon release, so then it is a good thing; as they seldom are vintage marked, you will know it is a fresh bottling. If it doesn't splay, it indicates it has spent some time in bottle, which means it just MAY have lost some of its freshness and fruitiness, and this is a bad thing.
If it is a Sparkler/Champagne intended for ageing (next to always with a vintage year on bottle) you will know by the vintage, and weather it splays or not is largely irrelevant.
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