It could be that the ezlink reader broke down, which explains
the free ride. However, such occurrence are rare as buses will not
be in revenue service if the reader malfunction.
Another possibility, which is the more likely one, is that you
chance upon a bus that promote certain products/services. It is a
new initiative by Moove Media, which uses buses as
lifestyle/product branding.
The list of buses that appears (in order of appearance that appear
on road):
1. Silkpro Bus
2. Silkair Bus
3. Nokia Music Bus
4. Guinness Bus
Out of the four, only Nokia Music Bus is still running around. I
think there are three of them in total. One of the bus is running
Service 156.