Well recently in all aircraft ive flown ive come across a slower plane than I am in but they can keep up with me can some 1 explain this please
Max speed given on charts is maximum sustainable speed in level flight at a given altitude. But you can easily get faster by by converting potential energy - you trade altitude for speed. How much faster you can get over your nominal maximum level speed, and how long you can sustain that "overspeed" depends on your plane's characteristics and your skill (always remember: "smooth is fast").
Also, while having the plane with higher top speed, you can still be catched in level flight by a slower plane that's simply better accelerating than you. A good example is P-51D vs Spit XVI:
Top speed at sea level: 369mph (P-51D) vs. 342 mph (Spitfire)
But acceleration 200 to 250 mph: 14 seconds (P-51) vs. only 11 seconds (Spitfire)