From the data you submit, the programme calculates three performance measures ('reliability', 'satisfaction' and 'efficiency') for each of four tank sizes.
A high 'reliability' means that for most of the year the householder will not need to fetch extra water from another source. A high 'satisfaction' means that most of the household's water can come from its RWH tank. A high 'efficiency' means that most of the roof run-off is being used, little is being wasted by overflowing the tank and we can say that performance is 'roof limited'.
Most owners of RWH systems have greatest interest in supply 'reliability', so that measure is generally the most useful of the three. RWH systems do not generally give 100% reliability, because to do so they would have to access a very large roof and contain an extremely large tank. Normally performance is either roof-limited or tank-limited. For most households it will be most economic to size the tank to give between 60% and 80% reliability and to buy or fetch water from other sources for part of each dry season.
To improve the performance of a RWH system there are four main options:
increase the tank size (however each time you double the tank size you increase system cost by about 60%);
reduce the nominal daily demand to well below mean daily runoff - (this will increase reliability but reduce the total water drawn and hence the system efficiency);
change to a more effective management strategy (from a fixed daily demand to one that is varied, with either tank water content or with season, to conserve water in the dry season);
increase the area of roofing that feeds the tank.
You should be able to visualise the effect of changing tank size by comparing the performance measures for the four tank sizes shown above.