the GTX 650 has less shaders(384 vs 768) and a higher clock rate the 650 ti has more(768) and a lower clock rate. the 650 ti is noticeably faster than the regular 650.
The boost is the newest member of the family allowing the 650 ti core to clock higher based on temperatures/power/ect. It also has a 192 bit memory controller vs the 128 bit on the other 2 cards. The boost also has faster memory from the factory.
Most mid level gpu's are just cut down versions of higher end ones.
They can take a 660 that did not have enough working shaders and make a 650 ti boost.
They can take a 650 ti boost that does is not good enough to boost and only has part of its memory controller working and make a 650 ti.
This is not uncommon. AMD's tri core cpus had all been quad cores with cores that did not work. At a point they had such demand that fully functional quad core cpus would have a core disabled just to get enough made. This lead to people unlocking cores on those cpus. The same has been done with some video cards in the past.
If they did not do this, the lower yields of some of the fabrication would make for too much waste. This way the can sell a GPU or CPU that did not make it as a XXX model but was fine for a YYY model.
No 2 gpus(or even cpus) are 100% the same this is also why boost speeds can be different(I have had to gtx 670's and one would boost to 1163 and the other would do 1215 by default) even with 2 of the same card purchased together. This also explains why some chips overclock better than others.
It is also important to know that boost will not be the same on all games as once you run out of power or get too hot it will drop a bit to get within the power/thermal limits.