Strange prices from Amadeus

I am checking the Amadeus about flight tickets and a little confused about the prices that I get. I expected to see prices from Amadeus closer to airline website prices or a little cheaper. But, I see that prices are incredibly high. For example, I made the following request:

https://api.amadeus.com/v2/shopping/flight-offers?originLocationCode=SGN&destinationLocationCode=DAD&departureDate=2020-05-29&adults=1&nonStop=true&currencyCode=VND

Flight at    Amadeus Price In USD   Airline price in USD
  11:45             ~70.41              ~38.72
  13:50             ~70.41              ~34.43
  18:00             ~70.41              ~42.93

Prices are higher significantly. Moreover, prices from Amadeus are the same for a lot of flights for the same air company.

Do I do something wrong or I skipped something?

PS: Response for first flight:

{
  "type": "flight-offer",
  "id": "1",
  "source": "GDS",
  "instantTicketingRequired": false,
  "nonHomogeneous": false,
  "oneWay": false,
  "lastTicketingDate": "2020-05-29",
  "numberOfBookableSeats": 4,
  "itineraries": [{
    "duration": "PT1H20M",
    "segments": [{
      "departure": {
        "iataCode": "SGN",
        "terminal": "1",
        "at": "2020-05-29T11:15:00"
      },
      "arrival": {
        "iataCode": "DAD",
        "terminal": "1",
        "at": "2020-05-29T12:35:00"
      },
      "carrierCode": "VJ",
      "number": "628",
      "aircraft": {
        "code": "321"
      },
      "operating": {
        "carrierCode": "VJ"
      },
      "duration": "PT1H20M",
      "id": "9",
      "numberOfStops": 0,
      "blacklistedInEU": false
    }]
  }],
  "price": {
    "currency": "VND",
    "total": "1640000.00",
    "base": "1381000.00",
    "fees": [{
        "amount": "0.00",
        "type": "SUPPLIER"
      },
      {
        "amount": "0.00",
        "type": "TICKETING"
      }
    ],
    "grandTotal": "1640000.00"
  },
  "pricingOptions": {
    "fareType": [
      "PUBLISHED"
    ],
    "includedCheckedBagsOnly": true
  },
  "validatingAirlineCodes": [],
  "travelerPricings": [{
    "travelerId": "1",
    "fareOption": "STANDARD",
    "travelerType": "ADULT",
    "price": {
      "currency": "VND",
      "total": "1640000.00",
      "base": "1381000.00"
    },
    "fareDetailsBySegment": [{
      "segmentId": "9",
      "cabin": "ECONOMY",
      "fareBasis": "JSP",
      "class": "J",
      "includedCheckedBags": {
        "weight": 20,
        "weightUnit": "KG"
      }
    }]
  }]
},

Solution

Keep in mind that right now the only source for prices in the API is GDS. Many LCCs don't / only load their more expensive fares into GDS, as it costs them money.

Lately, many "normal" airlines have also added fees when booking from a GDS compared to directly on their website / or other direct distribution methods.

I assume VietJet is one of those carriers, as it is a low-cost airline.

Regarding your second question: That is because of the fare system in the airline industry. Airlines load so called "fares", which specifies the price of a ticket for a given route within a date window.

An example: VietJet offers SGN-DAD for a base fare of 55EUR ( without taxes& fees ). You can choose any flight by VJ on any date within the window of the fare, and the price will be the same, as long as there is availability in the booking class J , which the fare is for.