If something is not guaranteed 100% of the time the guarantee is worthless.
Now I think we're disagreeing over the word "guarantee." Nothing in life is truly 100% guaranteed ever. By your definition of the term, no company should ever use the word "guarantee."
When you ship a package overnight via FedEx, it is guaranteed to arrive the next day and by a certain time. Doe every package always arrive on time? No. Occasionally, some don't.
At my work, our Internet provider guarantees a 99.999% uptime for their connections. Does that mean there is never a month where they have less than 99.999% uptime? No, occasionally something goes wrong and their uptime is lower.
A guarantee simply means that you are *supposed* to receive the promised product or service. When the guarantee is missed, the company is expected to compensate you. If FedEx misses a guaranteed delivery date, they will refund the shipping cost, for example. The fact that the remedy is written right into their policy shows that they acknowledge the possibility of failing to meet their promise. But it's still called a guarantee.
The bottom line is that Disney does not guarantee connecting rooms to most patrons and does not tell them they do. However, in this one circumstances, they *do* offer guaranteed connecting rooms. It's Disney policy, experienced travel agents have verified it, cast members from reservations have verified it, etc. You can prove it, even, by virtue of the fact that normally each room has a separate reservation number, but guaranteed connecting rooms are both booked under a single reservation number.
Again, nothing is 100% in life, but Disney will guarantee connecting rooms in this particular circumstance just the same as FedEx guarantees it's delivery times and Internet providers guarantee their uptimes.