I think this reflects the fact that originally they were classed as different types. The concept of differences training is quite valid when looking at moving from an S76A to an S76A+ - for instance - but becomes questionable in the case quoted.
The requirement is stated in Part FCL.710 and the UK CAA guidance (in CAP804) states this:
FCL.710 Type ratings – variants
(a) In order to extend his/her privileges to another variant of helicopter within type rating, the pilot shall undertake differences or familiarisation training. In the case of variants within a type rating, the differences or familiarisation training shall include the relevant elements defined in the operational suitability data established in accordance with Part-21.
(b) If the variant has not been flown within a period of 2 years following the differences training, further differences training or a proficiency check in that variant shall be required to maintain the privileges.
(c) The differences training shall be entered in the pilot’s
NOTE: There is no time stipulated, contrary to JR's comments....