The original .45-90 (WCF) was a Winchester developed express load w/ a 300 grn bullet: increased velocity plus a lighter bullet equaled a flatter trajectory.
According to Sellers, "The Sharps Firearms" (p. 340), the .45-2.4" Sharps straight, commonly & erroneously referred today as the .45-90, was introduced in 1877. The common load was w/ 100 grn of BP & 550 grn PP bullet.
Sellers further states that Sharps never loaded the 2.4" case w/ less than 100 grn of BP & it was designed as a target round.