It is because you are not counting the thousands, you are counting the trees.
So when you say:
There are seven thousand trees.
The noun trees is countable and plural. thousand is an adjective in this example. Even if you don't say "trees" its still implied.
But you could say:
There are thousands and thousands of trees."
where "thousand" is used as a noun and you are counting groups of 1,000.
Here's an edited version of a post I did for ELU on a similar question (which got closed):
The existential construction takes there as a subject. There has no meaning, and often the verb takes its agreement from the complement of the verb BE. So if the Noun Phrase after BE is plural, the verb will usually be in a plural form. If the Noun Phrase is singular it will usually be singular:
- There is an antelope over there.
- There are some antelopes over there.
Notice, however, that in the examples above, the subject and the verb BE are not contracted. In normal speech these will nearly always be contracted. We will use there's instead of there is. It is also quite common nowadays to see them contracted in writing, normally in informal texts, although you can find instances in prestigious newspapers like the Times, for example.
Now when the subject there and BE are contracted like this, the verb doesn't need to agree in any way with the following Noun Phrase. Therefore with regard to the Original Poster's example, if they had said:
*There is two options here.
... this sentence would be regarded as ungrammatical by most, if not all speakers. However because they did contract there and BE, it is grammatical:
There's two options here.
This makes this sentence similar to a famous lyric from one of John Lennon's songs:
Imagine there's no countries.
Or usages such as:
There's times when I've wanted to box his ears
Having said this, despite the fact that this is a well documented aspect of the grammar, some prescriptivists are bound to take offense at this. They will insist that it's ungrammatical to use a plural noun after there's. This will be despite the fact that they quite subconsciously actually use plural nouns after there's themselves quite frequently. They will appear about five minutes after I post this answer. They do make life fun though!
There are some other special situations where we might use there is with a plural noun phrase, even though it is not contracted. For example How many people live in your house? Well, there is me, my grandad, my mum and my aunt. If you'd like to read about these exceptions, there's some good posts here!.
Best Answer
The main issue is whether you are talking about an uncountable or a countable division of sugar between John and Tom. Uncountable nouns are singular:
Countable nouns are plural:
How do you decide which is right? Well, in this context, it makes more sense to assume you mean that John's sugar and Tom's sugar are of different varieties. Varieties of sugar are countable, so you would use the plural:
This could still be true even if talking about different defined quantities of sugar:
It might be a little confusing, but if it was obvious to the listener, you could leave out the exact measurement ("bowls of") and just say "sugars":
Lastly, just saying the sugars "are different" seems extraneous. Obviously they'd have to be, because the sugar belongs to different people. Instead you would say how they are different.
Both John's and Tom's sugars come from different countries.