Waste amount progress in St.Petersburg

I'm writing an article about waste management situation in St.Petersburg and Lenindrad region. Found some interesting sharpen decreasing of municipal solid waste collected in city in 2005 (original data provided to government by waste transporters). Amount of collected waste decreased by over 1.4 mil cubic meters. You can see it at fig. below. The question is why?

MSW collection, St.Petersburg

There are a lot of publications that points out dependence between amount of
waste and GDP. i.e. the more money you have the more waste you produce (consuming increases with incomes increasing). Were there any GRP or GRP per capita decreasing these years? Nope. Look at the figure below - it only grows.

GRP and GRP per capita in St.Petersburg

I've checked personal incomes for population - the trend is the same (but the numbers are lower by 30% in comparison with GRP per capita). Actually data about incomes is not very good presented in statistics - units aren't defined correctly so I decided not to create graph.

So the question "why?" remains. I can only conjecture:

1.  Changes in measurement system. As you may noticed, cubic meters are measuring units for waste amount. But volume is easily changeable as oppose to weight for solid waste (they measure weight in normal countries), so any changes in measurement affects result. For example they've started to measure weigh instead of volume and then calculate volume for statistics.
2. Changes in equipment. Transporters could start to use more trucks with compressing option - volume is easily changeable.
3. Illegal transportation and dumping. All this waste avoids statistics. There are over 200 licensed waste transporters (too much to control), but not every one acts completely legally.

Official statistics taken from here.