Swimming Tracking

I have a Suunto 9 Baro, and use it for many activities such as Climbing, Cycling, Surfing and Swimming.

The TSS score tends for swimming tends to be quite low (usually no greater than 40), even though I might burn >1000Kcal in about 50min, so there's usually barely any impact on my Fitness level. Is it normal that TSS scores are low for swimming? On other activities I usually easily go above 40 TSS in about 1h; even Yoga, for example, is usually higher although Kcal are much lower than swimming.

Maybe I am missing some knowledge here. If anyone can help me understand, let me know :)