C-13 NMR: Why are sp hybridized carbon atoms less shielded than sp3 hybridized carbon atoms but more shielded than sp2 carbon atoms?

For example, when you look at hex-1-en-5-yne, you'll find out that the terminal carbon with the triple bond has a reading of 69.7 ppm and the internal carbon with the triple bond has a reading of 86.1 ppm. This is significantly more downfield than the other sp3 carbons which have readings of 22.4 and 32.4 ppm, respectively. The other sp2 carbons have readings of 134.3 and 116.4 ppm, respectively. You can also see that the terminal carbons are more shielded, which makes me wonder why also they appear to be more electron rich? 


