This sub-polar low pressure belt is located between 60° and 70° latitudes in both the hemispheres. It is obvious that the low pressure is not caused by the temperature as there is low temperature throughout the year (Which should have caused high pressure). This shows that the low pressure belt is dynamically produced.
In the northern hemisphere wind from Sub-tropical high pressure region gets divided into two parts. One part blows towards the Equatorial Low Pressure Belt. The other part blows towards the sub polar low pressure belt. Because of the Coriolis effect the northern trade winds move away from the sub- tropical high in north-east direction. Winds coming from the sub-tropical and the polar high belts converge here to produce cyclonic storms or low pressure conditions.
Similarly in the southern hemisphere, the trade winds diverge out of the sub-tropical high belt and converge with polar high belt to produce low pressure conditions.
Edit: Two reasons for sub polar low pressure:
1.Due to the effect of axial rotation, the wind spreads outward from the polar region to sub polar area. This wind converges with the air coming from the sub-tropical area. The air rises up to create a low pressure zone.
2.Another reason is that, warm ocean currents from tropical and sub tropical area carry more heat to sub polar areas and create high temperature and low pressure.
As for horse latitude’s high pressure: The air rising at the equatorial belt starts blowing pole-ward. It converges with winds from polar regions at higher altitude above the sub-tropical zone. This results in sinking of air from higher altitudes. Thus descending winds decrease in their volume which ultimately causes high pressure.