Meaning of Cu1.5Al in Cu-Al phase diagram (I understand all other indexes incl. fractions like Cu0.75Al0.25)
As the page you referenced mentioned, things can get a bit tricky.  

I will start with your example of $\ce{Cu_{0.75}Al_{0.25}}$ - that is not just $75%$ Cu and $25%$ Al - that phase has a particular crystal structure, with the prototype (basically the first identified alloy with that crystal structure) of W (tungsten). So, it has the arrangement of atoms in the unit cell in the same places as pure tungsten, but $1/4$ of them are Al, and $3/4$ of them are Cu. There is no ordering of these atoms on the W lattice sites, or else the symmetry would be different, and the prototype might just be $\ce{Cu3Al}$.

For $\ce{Cu_{1.5}Al}$ it is similar, but perhaps even more confusing.  The 'Published phase label' is $\ce{Cu_{1.5}Al}$, so that is what it is usually called in the literature.  The 'formula' is $\ce{Cu_{1.7}Al}$, which just means when people started referring to the phase they didn't have an accurate composition measurement.  Then, the 'Prototype' is $\ce{Co_{1.75}Ge}$.  That means that the first similar crystal structure was measured in the Co-Ge system.  The non-integer subscript(s) indicate that the unit cell has vacancies in the ideal structure (without looking it up, I would speculate that the Co sites are $7/8$'s occupied). 

To further complicate matters, if you look at the Co-Ge phase diagram you will find that the phase label is now $\ce{Co_{1.67}Ge}$, Formula is $\ce{Co_{1.5}Ge}$, and the prototype is still called $\ce{Co_{1.75}Ge}$. 

So: crystal structures can be complicated, identification of composition has been getting better over time (folks have been looking at the Al-Cu system for a long time!), and if you really want to know what is going on you probably have to dive into the x-ray diffraction literature and look at the published atom positions in a given phase.