So you might wonder "why" the netspace is recently sometimes decreasing. In fact it is not decreasing, but this phenomen is a proof that the netspace is consistently slowing down its growing curve.
The netspace is calculated regarding the amount of proofs that are found compare to the difficulty. Every day the difficulty is adjusted to match the netspace such that there is always ~4608 block added to the blockchain.
However, for the difficulty adjustement to be relevant, it needs to include the growth rate of the Netspace, and not only a "snapshot", and it does. Until recently, the growing curve of the netspace, the derivative, was growing as well so we never noticed this "Sawtooth", but the derivative of the netspace might have stop growing. So what is happening ?
The netspace calculation algorithm take a snapshot of the growth rate and adjust the difficulty accordingly. But since this growth rate is decreasing, the algorithm "over-predict" the growth. It assume that the netspace will grow by X but in fact the netspace grow by Y with Y<X.
Basically, shortly before a difficulty adjustement the netspace is "over-estimated" because the previous adjustement has expected a certain growth, but the actual growth was lower. After an adjustement the netspace is re-evaluate to its real value, and the GUI show a decreasing Netspace. In fact, it apply a "correction". To be more precise, the difficulty is adjusted based on the previous growth rate, which is higher than the actual growth rate, so less proof than intended are found shortly after a difficulty adjustement, until the netspace reach the correct number for it to generate the expected amount of proofs on each challenge.
When the derivative of netspace was growing, this "correction" was not noticed, because the netspace was constantly predicted "lower" than it actually was, so the correction was not displaying a "decrease" but rather a "smaller increase". But these "sawtooth" we see recently indicate that netspace is now sometime calculated "higher" than it actually is. This is prooving that the netspace is slowing down and have reached an inflexion point.
Note that the netspace curve can reach multiple inflection point before really slowing down, especially with the pooling protocol imminent, it is expected that it will hit another inflexion point (derivative start growing again) before hitting a 3rd inflexion point and slow down definitively.