In fact, the 't' value is representative of an estimated landing weight (i.e. a force) not a mass. Examination of the (heliport manual) text referenced indicate that the 't' value should be based upon the worse conditions derived from consideration of the following two cases:
1.3.2.3 - the helicopter on landing; and
1.3.2.4 - the helicopter at rest.
I will not bore you with a repetition of the text contained in 'ICAO Doc 6261 Heliport Manual' but in the first section are included:
Dynamic load due to an impact on touchdown (which considers vertical speed at landing - both for the normal and emergency cases);
Increased loading on the FATO due to snow etc.;
Wind loading;
Punching shear (taking into account the likely loading at the point of contact - e.g. the wheels/skids);
The loading of the helicopter at rest also contains a number of elements - all of which are fairly logical.
Although there is debate on the subject, design of any helideck/heliport is usually based upon a 'design' helicopter, which is assumed to include the likely elements of the population of helicopters that are expected to use the facility. Authorisation has to be based upon the MCTOM of each helicopter because that is the only way that all expected masses can be considered.
Although the 'D' value represents an absolute limit - because it is based upon a invariable constant, the same is not true for 't' value, because of the assumptions made in the design process. In view of this, there is scope for States to conduct a safety case to consider helicopters with masses that exceed the original design 'mass' value. These studies are usually based upon installed power and assumptions made about the likely vertical speed of a helicopter that has had an engine failure on landing.
Unfortunately, these calculations are based upon the values that have been demonstrated during flight testing by the manufacturer - which are likely to be the Category A procedures. These may or may not be representative of the values that could be seen during an approach because they are likely to be outside the testing regime (exposure).
As with all of these questions, first recourse should be the the Operator - who should then explain the policy to the pilot. If there is doubt after this exchange, then an Occurrence Report might be filed (but this is quite a heavy tool to use).
Jim