Your hard disk spins at a constant rate, 7200RPM or whatever. the benchmark begins on the outside of the disk, where the radius is greater, and thus the linear speed is faster(one rotation in 1/120 of a second has greater distance (proportional to radius), and therefore more bits read in that time period), while inside the disk, the radius is smaller, and so less bits are read for the same angular distance(one rotation in 1/120 of a sec with smaller radius implies smaller circumference swept and thus less bits read.
Assuming an outside radius of about 2.8 in, and an inner radius of 1.6 in(due to loss for spindle, extra alignment space, landing zones), the performance loss on the inside is about a factor of 1.8.
Note that the jitter is caused by system load jitter or noise on cables, among other factors.
Also, addressing the SSD and not just the hard disk, is has an electronically network of connections set up(not mechanical) and thus the only delays are wire(in the actual memory) latency, and access "sweeps" across the data in blocks, keeping the velocity and bitrate constant, limited only by circuitry.