Weight gain or loss?

I thought it rather confusing, or misleading as to Nathan's weight gain or loss day to day. Usually, he weighs heavier when he wakes up in the morning, then as the day progresses, even after lunch and dinner, he would weigh lighter than the morning, which is really strange to our human calculated logic.

Yesterday, he clocked a good weigh gain of 10.5kg, both of us were delighted, saying well done, cos he's been hovering around 10kg since 9mths old and haven't had any significant growth. But this morning, when I checked, he lost 300g, straight after his big breakfast of oats and a cup of yogurt! What has he lost overnight? I'm not too sure about that...

We also measured his height last night, a very conventional way, using measuring tape. He's about 70cm tall only, gained roughly 21cm since birth, thought its pathetic. Hopefully, he gets taller than dad someday, that's our target for him.