Yup, it's extremely broken for software reasons. I see this across multiple Dells and ThinkPads. Linux has no problems sleeping on them. Windows just wakes up randomly on its own or never goes to sleep. Sleepstudy debugging doesn't show any reason. You can do some hacks to force it to hibernate instead of sleep to work around some of those problems.
A long time ago, when I went to I think the Event Manager or something, it showed a Bluetooth event when my laptop woke up while it was in my backpack. I don't know if the Bluetooth software caused it, or after it woke up it did a Bluetooth thing.
I imagine Event Manager or whatever still exists for those that want to investigate after it happens.
I no longer use Windows and use Ubuntu instead.