PairSet repetition in ttf font GPOS table - truetype

Question

I am reading ttf specification and realizing the parser. But I have encountered a problem of parsing 'GPOS' table.
This is the link to GPOS table specification: http://www.microsoft.com/typography/otspec/gpos.htm
It says:
ValueFormat2 applies to the ValueRecord of the second glyph in each pair. ValueRecords for all second glyphs must use ValueFormat2. If ValueFormat2 is set to null, then the second glyph of the pair is the “next” glyph for which a lookup should be performed.
My first question: What would happen when 'ValueFormat2' is 'null'? Is it the same with 'ValueFormat1'? If so, why the spec using different expressions?
Compared with the .ttx file generated by ttx, I find that there are repetitions of PairSet in GPOS section. But my parser couldn't read out such repetitions from ttf binary file. For example: times.ttf on Windows7. File 'time.ttx' has the following items, but my parser reads out 'index 13', then the next group is 'index 16', which means I have missed 'index 14' and 'index 15'. And I don't know why I cannot read out the same result as ttx does.
My second question: Is there anything I have missed from the ttf specification? Which and where are such repetitions explained? Thanks in advance!
<PairSet index="13">
<!-- PairValueCount=2 -->
<PairValueRecord index="0">
<SecondGlyph value="comma"/>
<Value1 XAdvance="-133"/>
</PairValueRecord>
<PairValueRecord index="1">
<SecondGlyph value="period"/>
<Value1 XAdvance="-133"/>
</PairValueRecord>
</PairSet>
<PairSet index="14">
<!-- PairValueCount=2 -->
<PairValueRecord index="0">
<SecondGlyph value="comma"/>
<Value1 XAdvance="-133"/>
</PairValueRecord>
<PairValueRecord index="1">
<SecondGlyph value="period"/>
<Value1 XAdvance="-133"/>
</PairValueRecord>
</PairSet>
<PairSet index="15">
<!-- PairValueCount=2 -->
<PairValueRecord index="0">
<SecondGlyph value="comma"/>
<Value1 XAdvance="-133"/>
</PairValueRecord>
<PairValueRecord index="1">
<SecondGlyph value="period"/>
<Value1 XAdvance="-133"/>
</PairValueRecord>
</PairSet>
<PairSet index="16">
<!-- PairValueCount=1 -->
<PairValueRecord index="0">
<SecondGlyph value="quoteleft"/>
<Value1 XAdvance="-152"/>
</PairValueRecord>
</PairSet>