Imagine that you are standing still next to a railroad track and a train is passing you at 60 mph. The boxcars are 1 mile long so that there is 1 minute between the arrival of each boxcar. You now start moving in the same direction that the train is traveling at 30 mph. You will notice 2 things:
1. The train is passing you more slowly.
2. The time between boxcars has increased to 2 minutes.
In May and November, the Earth is moving at "right angles" to the line to Algol. During this time we see minima happening regularly at their 2.867321 day intervals. However, during August, the Earth is rapidly moving towards Algol at about 107,229 km/hr as explained on my How Fast Are We Moving? page. (The Earth moves approximately 202 times its own size in one day.) So in 2.867321 days the Earth moves about 7,379,039 km closer to Algol. But the varying light from Algol doesn't know this - its light waves left Algol 93 years ago and are travelling at a constant speed. The result - we "catch a bunch of minima early" during August as shown on Chart 2. Exactly the opposite happens during February - the Earth is moving away from Algol that fast and it takes longer for the group of minima to reach us so we see them taking longer between events. How long? 7,379,039 km divided by the speed of light 299,792.458 km/sec is 24.61382 seconds - this rough calculation explains the deviations we see in Graph 2. So in May and November when we are not moving towards or away from Algol - the period seems constant. It is our rapid movement towards or away from the events in August and February that causes the timing differences.
In February, when the earth is moving away from Algol there is an increase in time between eclipses and the light is passing the earth more slowly just like when you increase your speed in the same direction that the train is traveling there is an increase in time between boxcars and the train is passing you more slowly.