A Random Number Generator, or RNG, is a computer algorithm that produces a pseudo-random number.
because computers cannot produce truly random numbers. Even when tied to the servers CPU clock, the behavior of a RNG can be mathematically predicted. That said, with thousands of players requesting numbers in a short time period, to the user it is as random as it needs to be.
A pseudo-random number generator has a tendency, from time to time, to "streak". That is, to produce a number of results that do not seem evenly distributed across the expected curve of results. This appearance is just that, an appearance, as over the long term the numbers will distribute evenly.