Using Realflo software, why the heating value can only be 1800 BTU/ft3 Maximum under AGA-8 Source (laboratory) ?

It's in the AGA-8 standard. According to Table 1: Ranges of Gas Mixture Characteristics Consistent with this Report  the expanded range for Gross Heating Value is 0 to 1800 BTU/scf
(where scf = standard cubic feet = ft3).

reference: AGA Report No. 8, Second edition, November 1992.

Released for: Schneider Electric USA