While there are some differences in U.S. process, it's also a matter of the terms of the treaty. You can read the text here.
The treaty enters into effect (See Articles 21 - 22) once it has been ratified, accepted, or approved by 50 states. Signing does not constitute acceptance or approval. However, a nation has to be a signatory before they can ratify, accept, or approve.
Signatories are not held to the terms of the treaty.
However, within the United States there are two important mechanisms for entering into international agreements.
- Treaties may only be entered into by the executive branch with the support of 2/3 of the members of the Senate. This is a Constitutional requirement.
- Executive Agreements can be entered into without congressional approval.
So what's the different? The President's authority is limited to a few situations, such as acts related to the armed services (which they are free to manage as the Commander in Chief).
In this case, signing the treaty did not obligate the United States to do anything, so the executive could sign it without congressional approval. However, ratifying the treaty would influence our domestic affairs, so it requires congressional approval.
The Wikipedia page on the Treaty Clause is a decent overview if you want more information.