First off, both of those processors are a 2010 vintage, and HEVC is a relatively new format, however it certainly doesn't require a dual core.
Neither are performing better, the graphics on the i3 is a 5 year old integrated chip, whereas the Pentium didn't have integrated, so the dedicated card from around the same time is out performing it, as it should be.
| Pentium T4500 | Core i3-380M
Release | Q1/2010 | Q3/2010
Cores | 2 | 2
Threads | 2 | 4
Speed | 2.3 GHz | 2.53 GHz
Graphics | None | Integrated (667 MHz)
Notably, the Intel Pentium T4500 is listed without a Graphics processor, but with two cores, whereas the Intel Core i3-380M is listed not only as two cores but running four threads. The slowdown is likely the integrated graphics on the i3-380M unit which by today's standards are ancient, and will likely have issues playing most HD video, regardless of format, although that is HEVCs primary use.
The reason the older, slower, and less threads (same amount of cores, 2) Pentium is winning out is because that model doesn't have integrated graphics, and as such will be using a dedicated card.