No idea why GDT pool payouts vary so much. I am staking 10k LSK and in tha middle of March I unvoted Joel and  voted in tony908.
These are the rewards from GDTpool since:

5.1 LSK  23rd March
4.9 LSK  30th March

4.8 LSK  6th  April
4.7 LSK  13th April
3.9 LSK  20th  April
5.2 LSK  27th April

4.5 LSK  4th May
4.4 LSK  11th May
4.3 LSK  18th May
4.2 LSK  25th May

4.3 LSK  1st June
4.2 LSK  8th  June
3.6 LSK  15th June
4.5 LSK  22nd June
3.9 LSK  29th JUne

3.9 LSK  6th July
3.7 LSK  13th JUly
3.7 LSK  20th July
3.7 LSk  27th July

2.1 LSK  3rd August



And the rewards from Lisk Elite from the same 10k LSK:


4.9 LSK  18th March
5.9 LSK  25th March

4.9 LSK  1st  April
5.9 LSK  8th  April
4.9 LSK  14th April
5.9 LSK  22nd April
4.9 LSk  29th April

5.9 LSK  5th  May
4.9 LSK  13th May
5.9 LSK  20th May
4.9 LSK  27th May

5.9 LSK  3rd June
4.9 LSK  10th June
4.9 LSK  17th JUne
5.9 LSK  24th June

4.9 LSK  1st  July
4.9 LSK  8th  July
5.9 LSK  15th July
4.9 LSK  21st July
4.9 LSK  29th July

As far as I know, there was no Lisk block reward halving nor any charges to additional delegates for GDT and Lisk Elite pools.
tony908 was not in the top 101 for a while, however since he got there my rewards decreased.
And looking at the rawards at both pools, I should be receiving around 5 LSK weekly from GTD.