I think you misinterpret the information given. On the very next FCOM page, there is some explanatory text and regulatory requirements to which the aircraft had been certified.
ENR 2 NM, Terminal 1 NM and APCH 0,5 NM (0,3 with GPS).
Aircraft itself estimates the probable positon error (based on available data sources and time from initialization / last update). This EPE value is evaluated against the certification requirements and pilot is warned whenever accuracy limit is not satisfied.
What you quote is a very in-depth knowledge on how exactly is the EPE calculated.
For instance the IRS mode with DME/DME updating the EPE may be as low as 0,28 NM. Essentially the manufacturer states that 0,28 NM is the best achievable precision in IRS/DME/DME mode. Why not 2 NM? Beacuse their equipment is much better than that. Why not 0,1 NM? I bet they wish but IRS/DME/DME installation just cannot do it.