Extra Spirit (T1 item) gives you 8 + 4 spirit power (12 total)
Improved Spirit (T3 item) gives you 21 + 12 spirit power (33 total)
Boundless Spirit (T4 item) gives you 60 + 16 spirit power (76 total)
Going from Improved Spirit to Boundless Spirit will show like this: 33 => 76, which at first doesn't make sense, but you have to understand that Boundless Spirit replaces Improved Spirit, so it doesn't just add 60 to your current Spirit Power.
Hope I explained it well enough.
(Same goes for health stat)