There are numerous sparkling wines made in France but not all of them are legally considered to be Champagne. To be Champagne, they must be produced within a specifically delineated region, the Champagne region. If they are not from that region, and even if they are made in the exact same way as a Champagne, they still cannot be labeled as Champagne.
Spanish sherry is very similar, as to be legally known as sherry, the wine must be produced within a specific region, the Denominación de Origen of Jerez-Xérès-Sherry, which located in the Andalucia region of southern Spain in the province of Cadiz. The “Sherry Triangle” is formed by the cities of Jerez de la Frontera, El Puerto de Santa María, and Sanlúcarde Barrameda.
You will find numerous people, websites, menus, and even books claiming that the wines of the Alvear winery are sherries. The 1927 Alvear Pedro Ximenez is a very popular wine, and the one most noted to be a sherry. It is easy to understand why it is thought to be so, as the method of production is essentially the same as sherry, with its use of the solera method. It will even taste like a sherry. But none of the Alvear wines can be legally known as sherry.
The Alvear winery is located in the D.O. of Montilla-Moriles, located in Andalucia but in the province of Cordoba. So it is not located within the "Sherry Triangle" and is not part of the D.O. Jerez-Xérès-Sherry. Thus, they cannot legally call their wines sherry. Just think of it as a sparkling wine made outside of Champagne, and thus unable to call itself Champagne.