Your basic contract is a 4 year contract with a pair of one (1) year extensions. These extensions are inactive extensions in that your current EAOS is 4 years from the day you reported to Active duty. These extensions remain inactive until you reach the point at which they begin, then they become active extensions. (hopefully that all made sense, it does sound rather wordy)
You are obligated to your first 1 year extension when you accept E-4 when graduating "A" School.
Second extension, when you go to prototype.
When you STAR at your 2 year point, you are serving a 4 year contract, with two years of inactive extensions. The calculation is based on the total number of months past your current EAOS you are giving the Navy. That's why the calculation only sees you having 2 years left. you are reenlisting for 6 years from that date.
Also, the calculation is actually (Base Pay)X(Multiple)X(Number of months added)/12. And the only reason you need to reenlist STAR is to get the automatic advancement to E-5 and a "guaranteed" "C" School. You can make E-5 off the exam, get paid for it (that's important because the base pay portion of the equation doesn't change until actually paid for higher rank), then reenlist for only 5 years and most likely max out the bonus.
Your Divisional Career Counselor should be able to tell you all of this stuff. If they can't, they aren't doing there job right and you need to see either the Departmental CC or your Command CC.
Best of luck and thank you for volunteering.