Aluminium is a very poor choice for electrodes. Its oxide layer makes erratic voltages. In addition, aluminium gets corroded also without an external current.
Pure aluminium eases the corrosion problem a bit but worsens the effectiveness of the oxide layer.
To my limited knowledge, no battery uses aluminium with an aqueous electrolyte. Aluminium batteries are scarce, I believe still in the research labs, and use exotic electrolytes.
Zn is much easier for the protective oxide layer despite it grows one too. But at commercial Zn batteries, an initial strong discharge current can be necessary to spoil the oxide layer before the cell gives its usual voltage. Possibly an explanation for your unexpected voltage - or not.
More generally, commercial batteries tend to use quite pure metals, like 99.9%. This does not change the output voltage, but it changes everything about the self-discharge, that is, the corrosion without an external current. Sometimes they use alloys for this same goal: decades ago, Zn was alloyed with about 0.1% Hg, which implied that batteries had to be collected to preserve the environment; presently, purer Zn avoids the addition of Hg.