To put it simply, at the price. Quality has its price. If I nachmesse, have (almost) all batteries, whether brand or the discounters the same voltage. Usually this is a new unused battery at 1.6 V. This voltage increases during the use from so far until the voltage is no longer sufficient that the machine is still running. But why is that that the batteries have different capacities? This is due to the internal resistance. If I want to have a good efficiency of a battery, or even the voltage drops below a certain minimum load voltage must not be less, so the connected equipment still works, it must have a low internal resistance. In order to avoid an undesirable over-proportional voltage drop, it is important that the internal resistance of the source is much lower than that of the consumer. In addition, the internal resistance increases through the discharge of the battery, that is just half full and now empty. A high internal resistance is also the reason why batteries are hot under load. Therefore, batteries with low internal resistance are the better choice for devices with high power requirements, as they have more power and not easily prone to voltage drop. Since this is not so easy to reach through a series of chemical processes, good batteries have hold their price.
For devices that do not have such a huge appetite for energy, it must not be the best, since it comes at low voltages to a more uniform discharge and not the voltage drop, the capacity but is sufficient. As already is added rather the self-discharge or the risk of leakage. But Duracell me are never leaked. So you have to have in mind rather with cheap batteries. For expensive equipment I'd risk anything. Whether the self-discharge does not begin until after 10 years, as Duracell promises, I could report only after 10 years. By then the batteries are empty but probably due to consumption. In this respect, the manufacturer can promise you.
Internal resistance lt. Datasheet of Duracells "Technical Library" (at the bottom of the web site) Plus Power: 120 mOhm Ultra Power: 81 mOhm, ie about 50% more power
