By default Safari wants to position any new tab opened after the last related tab. When you are browsing a site and open links from it in new tabs, having them immediately follow the current one is nice. However, when you simply want to open a new tab and have it appear at the end of the tab bar, you have to first position the focus on the current last tab, and then do CMD-T.
By enabling the “Debug” menu in Safari, you can access the “Tab Ordering” options. To enable the Debug menu run this command in the Terminal.
defaults write com.apple.Safari IncludeInternalDebugMenu 1
Then quit and reopen Safari. The Debug menu should be listed as the last menu in the Safari menu bar.
Locate the “Tab Ordering” menu option, and then hover over “Position of New Tabs”. There are four to choose from:
Click the option you prefer and your choice should take immediate effect.