Sigma patented a set of standard zoom lenses, experimenting with apertures from 2.8 to 4.0, and ranges as narrow as 28-70 to as long as 24-100. The news was first reported by Hi-Lows. Sigma has a long history of producing standard zooms that wind up not being quite as good as first party brands, an exception in their high-end lineup. The typical Sigma prime is both better in image quality and much cheaper in price than its OEM contemporaries.
Perhaps to address the historic weakness of its wider zoom offerings, Sigma appears to be testing out narrowed ranges to allow for more focused corrections. They did this previously with the now-classic 18-35mm f/1.8 Art lens for crop sensors, the design of which was brought over to full frame sensors with the 24-35mm f/2 Art a few years later. That latter lens was sharper at both 24mm and at 35mm than the Art primes in those focal lengths, but was widely criticized for having a short zoom range.
Of the several manifestations of the patent, the 28-70mm f/2.8 appears to be a market-acceptable range – especially now that Canon offers its own 28-70, although in a very large f/2 configuration. The others contemplated in the patent are a 28-100mm f/4; a 24-100mm f/4; a 20-80 f/2.8; and a second version of the 24-100mm f/4 that has two fewer elements and is five percent longer.