I can assume 2 variants:

1. This is the version 1 of the font.

2. The PostScript type of the font is 1:

Type 1 (also known as PostScript, PostScript Type 1, PS1, T1 or Adobe Type 1) is the font format for single-byte digital fonts for use with Adobe Type Manager software and with PostScript printers. It can support font hinting. It was originally a proprietary specification, but Adobe released the specification to third-party font manufacturers provided that all Type 1 fonts adhere to it.

I can assume 2 variants:

1. This is the version 1 of the font.

2. The PostScript type of the font is 1:

Type 1 (also known as PostScript, PostScript Type 1, PS1, T1 or Adobe Type 1) is the font format for single-byte digital fonts for use with Adobe Type Manager software and with PostScript printers. It can support font hinting. It was originally a proprietary specification, but Adobe released the specification to third-party font manufacturers provided that all Type 1 fonts adhere to it.

• I see I have no choice, so I give the right answer: For no reason. They just liked the word "ONE".