AWS Lambda和AWS Dynamodb:扫描计数小于项目计数

桌子 events 包含3700多个项目。当我扫描它时,它只返回791个项目计数。为什么?

Lambda

var doc = require('aws-sdk');
var db = new doc.DynamoDB();
exports.handler = (event, context, callback) => {
    var params = {
        TableName: "events",
        ProjectionExpression: "event, #date",
        FilterExpression: "event = :value",
        ExpressionAttributeNames: { "#date": "date" },
        ExpressionAttributeValues: { ":value": { S: "hurricane" } }
    };
    db.scan(params, function(err, data) {
        if (err) {
            console.log(err, err.stack);
        }
        else {
            console.log(data);
        }
    });
}

{项:[],计数:0,扫描计数:791,LastEvaluatedKey:{ 日期:{N:'1499875896175'}}}