It is pretty suprising!. There's no strong evolutionary reason for being able to taste the difference: there's no natural source of such concentrated D20 and it's only toxic if you ingest a lot of the concentrated stuff. It's not something any animal is going to encounter in nature.
And by conventional chemical wisdom D20 and H20 are virtually identical: the electron structure is the same and that basically dictates the chemistry. The only significant difference is the mass of the molecule (about 10% heavier), and experiments with oxygen-18 water (which has the same mass as D20) showed it doesn't taste of anything, so it must be due to very slight changes in structure between the two.
And by conventional chemical wisdom D20 and H20 are virtually identical: the electron structure is the same and that basically dictates the chemistry. The only significant difference is the mass of the molecule (about 10% heavier), and experiments with oxygen-18 water (which has the same mass as D20) showed it doesn't taste of anything, so it must be due to very slight changes in structure between the two.