This is what we educators call a teachable moment. Others have pointed out that it's the luck of the draw as to how far a given Pi can be overclocked, but you may be interested as to why that's the case. First, you should look at the word "overclock" - as the first two syllables suggest, overclocking means exceeding the rated speed of a device, which is 700 MHz, and no more, for the Pi. Any additional performance you may obtain is a statistical anomaly and this is true of any mass-manufactured device, whether it's based on a semiconductor process or anything else.
You may be surprised to find out that when any semiconductor device is produced that performs at a given level of performance (speed, in this case), it's one of a number (hundreds, possibly thousands) of other devices that may have higher, lower, or equal performance that can be in direct proximity to the device in question, usually on a semiconductor wafer. That's a very thin, circular disk up to 300 mm (~12inches) in diameter originally made of 99.9999999% pure silicon or other material with similar properties, such as germanium (first used for semiconductors due to its naturally-occurring properties) or gallium arsenide, used for extremely high-speed devices with high radiation immunity (also particularly fragile and expensive, only appropriate for highest-end military and space applications).
So, a single wafer, which is manufactured exactly like many thousands of others, actually has devices over the full range of possible levels of performance from worst to best. There aren't separate manufacturing lines for CPUs with lower, medium, and higher clock speeds in otherwise-identical models - they all come from the same manufacturing process. It's only through testing of each device that its performance level actually becomes known. Very few will reach the highest speed (around 1 GHz for the Broadcom BCM2835 used in the Pi) and a small number will only reach the rated 700 MHz, but probably half will perform at a speed of at least 850 MHz.
The statistical distribution of the performance of individual devices depends on many factors, including dozens to hundreds of discrete steps in the fabrication of each device, depending on its complexity. Any slight variance in temperature, time, material purity and concentration, etc., at the location of a device during a specific manufacturing step on a wafer can disproportionately affect the performance of that device. There is no way to predict what the performance of any particular device will be - it's strictly a numbers game and only testing can ascertain what attributes such as speed actually are.
Your mileage (per hour, or instruction cycles per second in this case) not only may vary, it will!
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!