iPhone XR no longer 128 gb

Both my mother and I bought iPhone XR’s a while back when they were new, we both got 128 gigabyte phones and went to check in the storage that they were both 128. Around a year later, when we went to check the storage of the phone, the phone was no longer capable of holding 128gb, but rather 64 gb. Meaning, the iPhone changed from being an 128 gb phone to a 64 gb phone. I do not see how this is possible, since storage capacity is built into the hard drive of the phone and not something that is able to change. What can I do about this?