The ctrl + tab shortcut is used to "quickly" switch between tabs in most - if not all - tabbed based browsers and other tabbed interfaces. The implementation is usually defaulted to Most Recently Used instead of sequential, but there is usually an option to select the behavior.

For example, if you have 3 tabs open - tab1, tab2, tab3 - and you currently are focusing on tab1. Next select tab3. Upon hitting ctrl + tab, tab1 should be selected. This works in every implementation, but in an MRU implementation, pressing ctrl + tab again would refocus tab3 and NOT tab2.

Chrome cycles through in order from left to right and would therefore focus tab2 in the situation above. A bug was open and Google's response was merely "won't fix" (sic). No period, no capitalization, not justification, just "won't fix".