Recently, I paid attention to the Rosh Chodesh Torah reading (Bamidbar/Numbers Chapter 28) and noticed that in verse 10, "olas Shabbas b'Shabato…", the word "Shabbas" is vowelized with a pasach (short o as in "pot") under the bais. Usually, the word has a kamatz (short u as in "dug") under the bais, being read "Shabbus", and transliterated as "Shabbos."
A pasach in this manner usually indicates a s'michus, meaning that the word is connected to the following word, typically as a description. Here, though, "Shabbas" does not appear to be describing the next word, "b'Shabato".
So why the unusual vowelization?